Frequency quadrupled laser using thulium-doped fiber amplifier and method

ABSTRACT

An apparatus, method and associated fiber-laser architectures for high-power pulsed operation and pumping wavelength-conversion devices. Some embodiments generate blue laser light by frequency quadrupling infrared (IR) light from Tm-doped gain fiber using non-linear wavelength conversion. Some embodiments use a fiber MOPA configuration to amplify a seed signal from a semiconductor laser or ring fiber laser. Some embodiments use the frequency-quadrupled blue light for underwater communications, imaging, and/or object and anomaly detection. Some embodiments amplitude modulate the IR seed signal to encode communication data sent to or from a submarine once the modulated light has its wavelength quartered. Other embodiments transmit blue-light pulses in a scanned pattern and detect scattered light to measure distances to objects in a raster-scanned underwater volume, which in turn are used to generate a data structure representing a three-dimensional rendition of the underwater scene being imaged for viewing by a person or for other software analysis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/799,982 filed Apr. 28, 2010, titled “HIGH-POWER LASER USINGTHULIUM-DOPED FIBER AMPLIFIER AND FREQUENCY QUADRUPLING FOR BLUE OUTPUT”(which issued as U.S. Pat. No. 8,953,647 on Feb. 10, 2015), which claimspriority benefit under 35 U.S.C. §119(e) to U.S. Provisional PatentApplication No. 61/198,405 filed on Oct. 13, 2009, titled “HIGH-POWERLASER USING THULIUM-DOPED FIBER AMPLIFIER AND FREQUENCY QUADRUPLING FORBLUE OUTPUT,” each of which is incorporated herein by reference in itsentirety.

This invention is related to:

-   -   U.S. Provisional Patent Application Ser. No. 60/896,265 filed on        Mar. 21, 2007, titled “HIGH-POWER, PULSED RING FIBER        OSCILLATOR”;    -   U.S. patent application Ser. No. 12/053,551 filed on Mar. 21,        2008, titled “HIGH-POWER, PULSED RING FIBER OSCILLATOR AND        METHOD” (which issued as U.S. Pat. No. 7,876,803 on Jan. 25,        2011);    -   U.S. patent application Ser. No. 12/050,937 filed Mar. 18, 2008,        titled “A METHOD AND MULTIPLE-MODE DEVICE FOR HIGH-POWER        SHORT-PULSE-LASER ABLATION AND CW CAUTERIZATION OF BODILY        TISSUES” (which issued as U.S. Pat. No. 8,202,268 on Jun. 19,        2012);    -   U.S. Pat. No. 7,429,734 titled “SYSTEM AND METHOD FOR AIRCRAFT        INFRARED COUNTERMEASURES TO MISSILES,” issued Sep. 30, 2008 to        Steven C. Tidwell, and filed Nov. 29, 2006;    -   U.S. Pat. No. 7,430,352 titled “MULTI-SEGMENT        PHOTONIC-CRYSTAL-ROD WAVEGUIDES FOR AMPLIFICATION OF HIGH-POWER        PULSED OPTICAL RADIATION AND ASSOCIATED METHOD,” issued Sep. 30,        2008 to Fabio Di Teodoro et al.;    -   U.S. Pat. No. 7,391,561 titled “FIBER- OR ROD-BASED OPTICAL        SOURCE FEATURING A LARGE-CORE, RARE-EARTH-DOPED PHOTONIC-CRYSTAL        DEVICE FOR GENERATION OF HIGH-POWER PULSED RADIATION AND        METHOD,” issued Jun. 24, 2008 to Fabio Di Teodoro et al., and        filed May 26, 2006;    -   U.S. patent application Ser. No. 12/018,193 titled “HIGH-ENERGY        EYE-SAFE PULSED FIBER AMPLIFIERS AND SOURCES OPERATING IN        ERBIUM′S L-BAND” filed Jan. 22, 2008 (which issued as U.S. Pat.        No. 7,872,794 on Jan. 18, 2011);    -   U.S. Pat. No. 7,620,077 titled “APPARATUS AND METHOD FOR PUMPING        AND OPERATING OPTICAL PARAMETRIC OSCILLATORS USING DFB FIBER        LASERS,” issued Nov. 17, 2009 to Angus J. Henderson;    -   U.S. Pat. No. 7,471,705 titled “ULTRAVIOLET LASER SYSTEM AND        METHOD HAVING WAVELENGTH IN THE 200-NM RANGE,” issued Dec. 30,        2008 and filed Nov. 9, 2006; and    -   U.S. Provisional Patent Application Ser. No. 61/263,736 titled        “Q-SWITCHED OSCILLATOR SEED-SOURCE FOR MOPA LASER ILLUMINATOR        METHOD AND APPARATUS,” filed by Savage-Leuchs et al. on Nov. 23,        2009; which are all incorporated herein in their entirety by        reference.

FIELD OF THE INVENTION

The invention relates generally to optical waveguides and moreparticularly to fiber lasers and fiber amplifiers that outputhigh-peak-power pulses (well over a kilowatt (kW)), in a first outputbeam having a first infrared signal wavelength (e.g., in someembodiments, about 1900 nm), wherein the first output beam is passedthrough a wavelength-conversion device to generate a second output beamat a second blue or blue-green wavelength that is one-quarter thewavelength of the first signal wavelength, near 475 nm, wherein, in someembodiments, the blue or blue-green output is used for communications toand/or from underwater locations and/or for underwater LIDAR (lightdistancing and ranging) or imaging where this output wavelength isparticularly beneficial for transmission through seawater.

BACKGROUND OF THE INVENTION

Conventional state-of-the-art lasers for outputting high-powerblue-green or blue light having wavelengths in the 450-500-nm region arevery costly, inefficient, and fragile. Prior-art devices for generatingthese wavelengths typically require many nonlinear conversion steps,including use of a tunable laser or optical parametric oscillator.

U.S. Pat. No. 6,288,835 titled “OPTICAL AMPLIFIERS AND LIGHT SOURCE” toLars Johan Albinsson Nilsson is incorporated herein by reference. Thispatent describes single- or few-moded waveguiding cladding-pumpedlasers, superfluorescent sources, and amplifiers, as well as lasers,including those for high-energy pulses, in which the interaction betweenthe waveguided light and a gain medium is substantially reduced. Thisleads to decreased losses of guided desired light as well as todecreased losses through emission of undesired light, compared todevices of the prior art. Furthermore, cross-talk and inter-symbolinterference in semiconductor amplifiers can be reduced. Also describedare devices with a predetermined saturation power, and a single(transverse) mode optical fiber laser or amplifier in which the activemedium (providing gain or saturable absorption) is shaped as a ring,situated in a region of the fiber's cross-section where the intensity ofthe signal light is substantially reduced compared to its peak value.The fiber can be cladding-pumped.

U.S. Pat. No. 4,867,558 titled “Method of remotely detecting submarinesusing a laser” that issued Sep. 19, 1989 to Leonard et al. and U.S. Pat.No. 4,893,924 titled “Method of remotely detecting submarines using alaser” that issued Jan. 16, 1990 also to Leonard et al. are bothincorporated herein by reference. Leonard et al. describe monitoringsubsurface water temperatures using a laser to detect subsurface wavesin a body of water such as an ocean caused by a submarine. A pulsedlaser beam is directed into the water to at least the depth of thethermocline and an analysis is made of the resultant Brillouin andRayleigh backscatter components. Wavelength shifted Brillouin scatter ismixed with the unshifted Rayleigh scatter in a self-heterodyne mannerfor each volume element of illuminated water, and the frequency of theheterodyne signal is measured and converted into temperature. In thosepatents, the scheme is not directly detecting the submarine but insteadis detecting the internal waves in the thermocline boundary in theseawater. The submarine's passage leaves ripples in the thermocline,which are subsequently detected by the system incorporating a laser.

U.S. Pat. No. 7,283,426 titled “Method and apparatus for detectingsubmarines” that issued to Grasso on Oct. 16, 2007 is incorporatedherein by reference. Grasso describes detecting, tracking and locatingsubmarines utilizing pulsed coherent radiation from a laser that isprojected down through a water column, with particles in the waterproducing speckle from backscatter of the random particle distribution,with correlation of two closely time-spaced particle-based specklepatterns providing an intensity measurement indicative of the presenceof a submarine. Subsurface submarine movement provides a subsurface wakewhich causes movement of particles such that two closely-spaced“snapshots” of the returns from particles in the same water column candetect particle movement due to the wake.

U.S. Pat. No. 5,270,780 titled “Dual detector LIDAR system and method”that issued to Moran et al. on Dec. 14, 1993 is incorporated herein byreference. This patent describes a light detection and ranging (LIDAR)system that uses dual detectors to provide three-dimensional imaging ofunderwater objects (or other objects hidden by a partially transmissivemedium). An initial laser pulse is transmitted to known x-y coordinatesof a target area. The photo signals returned from the target area fromthis initial pulse are directed to the low resolution, high bandwidthdetector, where a preliminary determination as to the location (depth,or z coordinate) of an object in the target area is made based on thetime-of-receipt of the return photo signal. A second laser pulse is thentransmitted to the target area and the return photo signals from suchsecond laser pulse are directed to the high resolution, narrow bandwidthdetector. This high-resolution detector is gated on at a time so thatonly photo signals returned from a narrow “slice” of the target area(corresponding to the previously detected depth of the object) arereceived.

U.S. Pat. No. 5,504,719 titled “Laser hydrophone and virtual array oflaser hydrophones” that issued to Jacobs on Apr. 2, 1996 is incorporatedherein by reference. This patent describes a hydrophone or a virtualarray of hydrophones for sensing the amplitude, frequency, and inarrays, the direction of sonic waves in water. The hydrophone employs alaser beam which is focused upon a small “focal” volume of water inwhich natural light scattering matter is suspended and which mattervibrates in synchronism with any sonic waves present. The vibrationproduces a phase modulation of the scattered light which may berecovered by optical heterodyne and sensitive phase detectiontechniques. The sonic waves are sensed at locations displaced from thefocusing lenses.

U.S. Pat. No. 5,091,778 titled “Imaging LIDAR systems and K-metersemploying tunable and fixed frequency laser transmitters” that issued toKeeler on Feb. 25, 1992 is incorporated herein by reference. Keelerdescribes a laser imaging system for underwater use that employs awavelength-tunable laser. In particular, Keeler emphasizes the operationof the laser at blue wavelengths to optimize the performance in the openocean.

U.S. Pat. No. 7,505,366 titled “Method for linear optoacousticcommunication and optimization” that issued to Blackmon et al. on Mar.17, 2009 is incorporated herein by reference. Blackmon et al. describeoptical-to-acoustic energy conversion for optoacoustic communicationfrom an in-air platform to an undersea vehicle. They describe directinga high-power laser at the ocean surface in order to generate acousticwaves (sound), wherein the sound is used as the communications signal toan underwater target receiver. Blackmon et al. assert that signals usedin underwater acoustic telemetry applications are capable of beinggenerated through a linear optoacoustic regime conversion process. Theyaddress the use of oblique laser beam incidence at an air-waterinterface to obtain considerable in-air range from the laser source tothe water surface where the sound is formed to communicate to theundersea vehicle.

U.S. Patent Application Publication 2007/0253453 titled “Solid-statelaser arrays using” published Nov. 1, 2007, and U.S. Patent ApplicationPublication 2008/0317072 titled “Compact solid-state laser” publishedDec. 25, 2008 both by Essaian and Shchegrov, are incorporated herein byreference. Essaian et al. describe a compact solid-state laser array fornonlinear intracavity frequency conversion into desired wavelengthsusing periodically poled nonlinear crystals. The crystals containdopants such as MgO and/or have a specified stoichiometry. Oneembodiment includes a periodically poled nonlinear crystal chip such asperiodically poled, MgO-doped lithium niobate (PPMgOLN), periodicallypoled, MgO-doped lithium tantalate (PPMgOLT), periodically poled,ZnO-doped lithium niobate (PPZnOLN), periodically poled, ZnO-dopedlithium tantalate (PPZnOLT), periodically poled stoichiometric lithiumniobate (PPSLN), and periodically poled stoichiometric lithium tantalate(PPSLT), periodically poled MgO- and ZnO-doped near-stoichiometriclithium niobate (PPMgOSLN, PPZnOSLN), or periodically poled MgO- and/orZnO-doped near-stoichiometric lithium tantalate (PPMgOSLT, PPZnOSLT),for efficient frequency doubling of an infrared laser pump beam into thevisible wavelength range. The described designs are said to beespecially advantageous for obtaining low-cost green and blue lasersources. The use of such high-efficiency pumps and nonlinear materialsallows scaling of a compact, low-cost architecture to provide highoutput power levels in the blue/green wavelength range.

What are needed are improved methods and apparatus for generatinghigh-power pulses of infrared (IR) light of particular wavelengths andconverting this light to blue-green and/or blue light. Also needed aresystems capable of deep underwater communications, imaging, and othersensing using light obtained from a frequency-converted laser beam.

SUMMARY OF THE INVENTION

In some embodiments, the present invention provides an apparatus, methodand associated fiber-laser architectures for high-power pulsed operationand for pumping of wavelength-conversion devices. In some embodiments,the wavelength conversion generates blue laser light by frequencyquadrupling the infrared light from an initial thulium (Tm)-doped fiberlaser using a non-linear wavelength conversion device. In someembodiments, the initial Tm-doped fiber laser uses a master-oscillatorpower-amplifier (MOPA) configuration that uses a seed laser beam from asemiconductor laser that is amplified by one or more fiber amplifiers.In other embodiments, the initial laser (or the seed laser if theinitial laser uses a MOPA configuration) is a Q-switched orcavity-dumped ring fiber laser. In yet other embodiments, the seedsource includes a distributed feedback (DFB) laser diode, a distributedBragg reflector (DBR) laser diode, or a laser diode externallystabilized with a fiber Bragg grating or a volume Bragg grating.

In some embodiments, the one or more fiber amplifiers include alarge-mode-area (LMA) fiber and/or include a polarization-maintaining(PM) fiber and/or include a multiply-clad fiber that uses claddingpumping and/or uses a plurality of stages (e.g., lengths of active(amplifying) fibers separated by “pump blocks” (e.g., monolithicfree-space optical elements that inject additional pump light and/orfilter the signal light to narrow the linewidth and/or remove amplifiedspontaneous emission (ASE) and then pass the signal light to a furtheramplifying fiber such as described in commonly assigned U.S. Pat. No.7,537,395 titled DIODE-LASER-PUMP MODULE WITH INTEGRATED SIGNAL PORTSFOR PUMPING AMPLIFYING FIBERS AND METHOD that issued May 26, 2009, andas described in commonly assigned U.S. patent application Ser. No.11/420,751 that was filed May 27, 2006 (which issued as U.S. Pat. No.7,941,019 on May 10, 2011) titled MONOLITHIC PUMP BLOCK FOR OPTICALAMPLIFIERS AND ASSOCIATED METHOD, and which are each incorporated hereinby reference))).

In some embodiments, the frequency-quadrupled blue light from the laseris used for underwater communications, imaging, and/or object andanomaly detection. In some embodiments, the infrared (IR) light from theinitial or seed laser is pulsed and/or otherwise amplitude modulated,wherein the pulses and/or other amplitude modulation encode data that isto be communicated to or from an underwater vehicle (such as asubmarine) once the modulated light has its wavelength quartered (i.e.,has its frequency quadrupled). In other embodiments, thefrequency-quadrupled blue-light pulses are transmitted in a scannedpattern (such as a raster scan) and a light detector measuresreflections of the light pulses to allow time-of-flight measurement ofdistances to objects or other anomalies in each of a plurality ofdirections (i.e., of the raster-scanned underwater volume), which inturn are used to generate a data structure representing athree-dimensional rendition of the underwater volume (i.e., of the scenebeing imaged) for viewing by a person or for other software processingand analysis.

The architectures of the present invention enable operation of theinitial or seed fiber laser in Q-switched, cavity-dumping, or hybridQ-switched/cavity-dumping modes. In all of these modes of operation, theinitial or seed laser is designed as a unidirectional ring cavity, whichminimizes pulse-to-pulse amplitude/temporal instabilities and feedbackeffects.

In some embodiments of the Q-switched mode, the initial or seed laserincludes a large-core rare-earth-doped fiber featuring a core having alow numeric aperture (NA) (in some embodiments, the low core NA isexplicitly configured and intended to minimize the fraction ofspontaneous emission from the active species (e.g., the dopant) that iscaptured and amplified in the core), an electro-optic switch of highon/off extinction (10 dB or higher) that provides enough inter-pulseextinction to minimize circulation and amplification of spontaneousemission in the cavity of the initial or seed laser (in one of theinvention's baseline embodiments, this modulator is a small-aperturerubidium titanyl phosphate (RTP) Pockels cell), an output coupler, andan intracavity bandpass filter to enforce lasing operation in a narrowwavelength range.

In some embodiments of the cavity-dumped mode, the initial or seed laseris configured in a similar manner, except that an output coupler is nolonger necessary, since the optical power can be extracted from thelaser cavity by the electro-optic switch itself. The same initial orseed laser can be configured to operate in both Q-switched andcavity-dumping modes as well as in hybrid modes (e.g., partial Q-switch,followed by cavity dumping). In some embodiments, the initial or seedlaser can be used as, or inject laser light into, a regenerativesolid-state optical amplifier.

Some embodiments include an all-fiber pulsed or Q-switched ring laser(such as described in U.S. Provisional Patent Application 61/263,736titled “Q-SWITCHED OSCILLATOR SEED-SOURCE FOR MOPA LASER ILLUMINATORMETHOD AND APPARATUS,” filed by Savage-Leuchs et al. on Nov. 23, 2009,which is incorporated herein by reference). Other embodiments use adifferent type of ring laser as the initial master-oscillator or seedstage in a master-oscillator power-amplifier (MOPA) system, or as apower-oscillator stage, the ring laser having a large-corerare-earth-doped fiber that is ring-connected with a free-space pathhaving an electro-optic switch, output coupler, one-way (unidirectional)isolator and/or intracavity bandpass filter to enforce lasing operationin a narrow wavelength range (such as described in U.S. patentapplication Ser. No. 12/053,551, titled “HIGH-POWER, PULSED RING FIBEROSCILLATOR AND METHOD” filed by Di Teodoro et al. on Mar. 28, 2008,which is incorporated herein by reference, and which issued as U.S. Pat.No. 7,876,803 on Jan. 25, 2011). In some cavity-dumped modes, the laseris configured in a similar manner, except that an output coupler is nolonger necessary, since the optical power can be extracted from thelaser cavity by the electro-optic switch itself. In some embodiments,the same laser is configured to operate in Q-switched, cavity-dumpingmodes, in hybrid modes (e.g., partial Q-switch, followed by cavitydumping), or even continuous wave (CW; i.e., a laser beam that iscontinuous and substantially constant in amplitude when the laser is on,and not pulsed or amplitude modulated) wherein some downstream outputbeam is amplitude modulated. In some embodiments, the laser is used as,or injects laser light into, a regenerative solid-state amplifier, isused as a Raman amplifier, or is used as a Raman laser to accesswavelengths in the near- and mid-infrared wavelength ranges, whichwavelengths are then wavelength converted to one-quarter the wavelengthusing non-linear wavelength conversion. In some embodiments, the laseris also used to generate visible, ultra-violet, mid-infrared, andfar-infrared terahertz (THz) radiation via nonlinearwavelength-conversion processes including frequency doubling, triplingand quadrupling; optical-parametric generation, optical-parametricamplification, and optical-parametric oscillation; difference-frequencymixing; sum-frequency mixing; and optical rectification. In some of anyof these embodiments, the initial IR laser is used as a stand-alonelaser whose output is wavelength converted to a wavelength in the rangeof 450-500 nm, while in other embodiments, the initial IR laser is usedas a seed or master laser for one or more optical power amplifiers (themaster-oscillator power-amplifier (MOPA) configuration), and the outputof those one or more amplifiers is wavelength converted.

In some embodiments, the initial laser of the present invention, in allof its modes of operation (Q-switched, cavity-dumped, or partialQ-switch followed by cavity dumping), emits radiation in the 1650- to2100-nm range (in some embodiments, using a fiber doped with Tm, Ho, orboth) (and the final output wavelength is in the 412- to 525-nm range).In some embodiments, the wavelength is in the 1800- to 2000-nm range(and the final output wavelength is in the 450- to 500-nm range). In allof these embodiments, the initial laser can be used as a stand-alonelaser whose output frequency is quadrupled, or as a seed laser foroptical amplifiers whose output frequency is quadrupled.

In some embodiments, the present invention provides high-power outputpulses that are used for underwater communications, or for detection ofunderwater objects or disturbances (such as turbulence due to submarinesor marine animals), or for mapping sea-bottom topography.

Other advantages of the present invention include low cost, relativelycompact footprint, few parts, solid-state parts, and relatively simplesetup and operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective block diagram of a visible-light communicationssystem 101 using a frequency-quadrupled IR laser system 190 that outputsand/or detects light in a wavelength range of about 450 nm to about 500nm.

FIG. 1B is a block diagram of a visible-light communications system 102using a frequency-quadrupled IR laser system 190 that outputs and/ordetects light in a wavelength range of about 450 nm to about 500 nm.

FIG. 1C is a perspective block diagram of a visible-lightsensing/imaging system 103 using a frequency-quadrupled IR laser system193 that outputs and/or detects light in a wavelength range of about 450nm to about 500 nm.

FIG. 1D is a block diagram of a visible-light sensing/imaging system 104using a frequency-quadrupled IR laser system 193 that outputs and/ordetects light in a wavelength range of about 450 nm to about 500 nm.

FIG. 2A is a block diagram of a visible-output system 201 using an IRmaster-oscillator power-amplifier (MOPA) source laser 210A andwavelength converter 250.

FIG. 2B is a block diagram of an IR-output system 202 using an IR ringlaser 210B.

FIG. 3A is a block diagram of a one-or-more-wavelength-output system 301using an IR laser 310A.

FIG. 3B is a block diagram of a one-or-more-wavelength-output system 302using an IR laser 310B.

FIG. 4 is a graph 401 showing the spectrum of sunlight.

FIG. 5A is a series of graphs 501 showing sunlight penetration atvarious depths in seawater.

FIGS. 5B-5C are graphs 503-504 showing polarization differences of lightin seawater.

FIG. 6 is a block diagram of a visible-output system 601 using an IRring laser 610.

FIG. 7A is a block diagram of a multiple-wavelength-output system 701using an IR ring laser 710.

FIG. 7B is a graph 703 of an electrical-pulse waveform 741.

FIG. 7C is a graph 704 of an optical-pulse waveform 742.

FIG. 8 is a block diagram of a materials-processing system 800 using oneor more of the ring-laser systems 811 as described herein.

DETAILED DESCRIPTION OF THE INVENTION

Although the following detailed description contains many specifics forthe purpose of illustration, a person of ordinary skill in the art willappreciate that many variations and alterations to the following detailsare within the scope of the invention. Accordingly, the followingpreferred embodiments of the invention are set forth without any loss ofgenerality to, and without imposing limitations upon the claimedinvention. Further, in the following detailed description of thepreferred embodiments, reference is made to the accompanying drawingsthat form a part hereof, and in which are shown by way of illustrationspecific embodiments in which the invention may be practiced. It isunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

The leading digit(s) of reference numbers appearing in the Figuresgenerally corresponds to the Figure number in which that component isfirst introduced, such that the same reference number is used throughoutto refer to an identical component that appears in multiple figures.Signals and connections may be referred to by the same reference numberor label, and the actual meaning will be clear from its use in thecontext of the description.

In some embodiments, the present invention provides an apparatus andprocess wherein a fiber ring laser is operated in Q-switched,cavity-dumping, or hybrid Q-switched/cavity-dumping modes. In someembodiments, the fiber laser operates in the infrared wavelengths(having one or more wavelengths selectable via the one or more activedopant species that is/are used in the fabrication of the fiber, and viathe bandpass filter(s) used), and outputs short-duration very-high-powerpulses (e.g., thousands of kilowatts, in some embodiments). In someembodiments, the output of the fiber ring laser is converted toradiation having a desired wavelength in the visible or ultra-violet fortransmission through water, and in particular seawater, via nonlinearwavelength-conversion devices including frequency doubling, tripling,quadrupling and/or quintupling elements, optical-parametric generationunits, optical-parametric amplification units, and optical-parametricoscillation units, difference-frequency mixing units, sum-frequencymixing units, and optical-rectification units, for example. Someembodiments also output radiation in the mid-infrared or far-infrared(THz) range for use in communication in air between aircraft and/orsurface vessels.

FIG. 1A is a perspective block diagram of a visible-light communicationssystem 101 using a frequency-quadrupled IR laser system 190 that outputsand/or detects light in a wavelength range of about 450 nm to about 500nm (generally perceived by humans as blue colors), according to someembodiments. In some embodiments, the initial source infrared (IR) laserhas a wavelength of about 1800 nm to about 2000 nm. In some embodiments,an underwater vehicle 616 is in one-way or two-way communications with aremote vehicle (such as another underwater vehicle 116 (such as asubmarine), a surface vessel 47 (such as a destroyer, missile cruiser oraircraft carrier) and/or an aircraft 48 (such as a helicopter or jetfighter) or a land vehicle (such as a HUMVEE®, not shown here (“HUMVEE”is a registered trademark of AM General Corp.)). Because green colorspropagate better in the types of water found near shore, someembodiments mounted on a land vehicle use a frequency-quadrupled IRlaser system 190 that outputs and/or detects light in a wavelength rangeof about 500 nm to about 550 nm (generally perceived by humans as greencolors), and the initial source IR laser has a wavelength of about 2000nm to about 2200 nm. In some embodiments, each such vehicle includes atransmitter and/or receiver portion of a communications system 190connected by fiber-optic cables within the vehicle to and/or from avehicle-to-external interface 191 (e.g., in some embodiments, afiber-optic array and transform lens such as described in commonlyassigned U.S. Pat. No. 7,429,734 titled “SYSTEM AND METHOD FOR AIRCRAFTINFRARED COUNTERMEASURES TO MISSILES,” which issued to Steven C. TidwellSep. 30, 2008, which is incorporated herein by reference; or alaser-pointing turret or gimbal; or, in other embodiments, some othersuitable laser-output port to the vehicle, such as a fixed wide-angletransmission lens having little directionality in order to not show anenemy observer which direction the communications target is located andthus to disguise or not reveal the location of the underwater target ofthe communications). In some embodiments, the transmitted laser beam 199from each vehicle has a wavelength of approximately 470 nm (0.47microns) or other suitable wavelength to which seawater is relativelytransparent, and is modulated (e.g., using amplitude-modulation orpulse-width modulation or other suitable modulation scheme) with data(e.g., text, images, or other data, which may or may not be encrypted,as desired by the parties to the communication) that is to becommunicated. In some embodiments, the communications system 101 isconsidered to include one or more of the vehicles 47, 48, 116, and/or616, themselves, to which the data-and-laser systems 190 are mounted.The seawater surface (water-air interface) is labeled with referencenumber 89.

FIG. 1B is a block diagram of a visible-light communications system 102using a frequency-quadrupled IR laser transmission system 190 thatoutputs an encoded data stream that is modulated onto the transmittedoutput laser beam 199 having a narrowband (e.g., in some embodiments, aFWHM (full-width half-maximum) linewidth of 1 nm or less) lightwavelength in a wavelength range of about 450 nm to about 500 nm (thisoutput laser beam 199 is transmitted to a remote vehicle 117) and/or areceiver-detector system 150 that detects light in a wavelength range ofabout 450 nm to about 500 nm (this is received from remote vehicle 117),and demodulates the encoded data 159 which is output and/or stored. Insome embodiments, the receiver-detector system 150 includes a filter 156(in some embodiments, a narrowband wavelength filter and/or polarizationfilter) to essentially eliminate all wavelengths (and/or polarizations)other than the wavelength (and/or polarization) of the narrowband(and/or polarized) transmitted light 199, in order to increase thesignal-to-noise ratio. In some embodiments, the data 149 that is to becommunicated is supplied to communications-transmission modulator 151which controls a modulation of the laser light generated by laser 152(in some embodiments, this includes a thulium-doped fiber laser and/orthulium-doped fiber amplifier as described below). In some embodiments,the modulated and amplified intermediate laser beam has a wavelength ofapproximately 1880 nm (1.88 microns) that is coupled to frequencyquadrupler 153 (which converts the wavelength to 25% of the originalwavelength, which is 470 nm (0.47 microns)). In some embodiments, this470-nm output beam (still having the data encoded on it) is directed ina particular direction by beam-steering unit 154. In some embodiments,beam-steering unit 154 is the external interface 191 of FIG. 1A. Thisbeam steering directs a majority of the output beam toward the desiredtarget receiver, in order to further improve signal strength and thesignal-to-noise ratio of the signal. The portion of this signal 199 thatis received by the remote target vehicle 117 (i.e., a vehicle at adistance from the transmitter vehicle) is processed as described in thenext paragraph (in the description of the receiving apparatus 150). Insome embodiments, the remote vehicle 117 also transmits encoded data ona laser beam that it generates and transmits in a manner as describedearlier in this paragraph, using its own transmitting apparatus 190.

In some embodiments, each vehicle in system 102 of FIG. 1B has areceiving apparatus 150 configured to receive laser-beam communicationsfrom other vehicles. For example, in a surface ship (vehicle) 47 (seeFIG. 1A), the laser beam 199 from the remote vehicle 117 is received(e.g., by an optional receiver pointer unit 155 that preferentiallyreceives laser light of a particular wavelength (e.g., 470 nm) from aparticular direction (e.g., from the direction of the remote vehicle117) in order to increase its signal-to-noise ratio. In someembodiments, the received laser signal and any associated light noise(e.g., other wavelengths that are not desired) is passed throughnarrowband wavelength bandpass filter 156 that passes only the desiredwavelengths (e.g., having a FWHM linewidth of 1 nm or less, centered ata wavelength of 470 nm) and rejects other wavelengths. This filteredlight is then detected by beam detector 157 which generates anelectrical signal representative of the encoded data, and thiselectrical signal is coupled to communications-reception demodulator158, which then outputs the decoded data 159 (e.g., text, images, orother tactical or strategic data). In some embodiments, thebeam-steering unit 154 and/or the receiver-pointer unit 155 are includedin the vehicle-interface unit 191 of FIG. 1A.

In some embodiments, the transmitted beam 199 is polarized (e.g., insome embodiments, linearly polarized; in other embodiments, circularlypolarized). In some embodiments, filter 156 of receiver 150 includes apolarizer having an orientation that is, or selectively can be, orientedto match the polarization of the transmitted beam 199. In otherembodiments, receiver 150 is replicated in whole or in part, wherein areceived light signal that includes beam 199 is directed through apolarizing beam splitter (considered part of filter 156), e.g., suchthat the horizontal polarized light is split from the verticallypolarized light (in other embodiments, the incoming beam is split orreceived by different receiver pointers and each part directed through aseparate polarized filter 156 having a different polarization). In someembodiments, the different polarizations are each detected and theresulting signals subtracted from one another in order to furtherdistinguish the desired signal having one polarization from the portionof background light having a different polarization.

FIG. 1C is a perspective block diagram of a visible-lightsensing/imaging system 103 using a frequency-quadrupled IR laser system193 that outputs and/or detects light in a wavelength range of about 450nm to about 500 nm. In some embodiments, system 103 transmits an outputlaser beam 197 that is used to illuminate an underwater scene 143 and/or144 (e.g., a pulsed beam that when reflected can be used for determiningranges and directions to various objects). In some embodiments, apassing submarine or animal causes turbulence 142 that in turn causes adisturbance 143 to a neighboring thermocline 141. The transmitted laserbeam 197 is reflected by disturbance 143 and the reflected light 196 isthen detected and/or imaged via laser-interface unit 194 (e.g., attachedto surface vessel 47, aircraft 48 or even another underwater vehiclesuch as vehicle 616 of FIG. 1A). In some embodiments, the transmittedlaser beam 197 (which is a water-source to water-destination beam) ispulsed, and reflections from the water surface (in the case of aerialvehicles) and detection of the reflected pulse from various underwaterfeatures (such as thermoclines, sea life, and bottom surface) allowstime-of-flight measurements that are combined with the angular directionof where the transmitted pulse was directed, and/or from which thereflected pulse was detected, to provide three-dimensional (3D)information from which a 3D image (e.g., one that is generated as twoimages to be viewable with stereoscope glasses to the left eye and righteye of a person viewing the 3D image) or a two-dimensional (2D) image(e.g., a 2D image where the viewpoint of the 3D data is rotatable(varying the azimuth and altitude angles) and zoomable (varying thedistance from the 3D features) by the viewer using conventional 3Dviewing software (such as software browsers capable of rendering X3Dmarkup language (which is the ISO-standard XML-based file format forrepresenting three-dimensional (3D) computer graphics), or VirtualReality Modeling Language (VRML); the X3D standard features extensionsto VRML (e.g., Humanoid Animation, NURBS (Non-Uniform RationalB-Splines, which are mathematical representations of 3D geometry thatcan accurately describe any shape from a simple 2D line, circle, arc, orcurve to a very complex 3D organic free-form surface or solid), GeoVRML(VRML for the representation of geographical data), and the like), andenhanced application programming interfaces (APIs). In some embodiments,the scene being imaged includes underwater surface topology 144. In someembodiments, the output laser beam is focussed to a narrow beam that isscanned (e.g., in two angular directions to perform an X-Y raster scanor other suitable scan pattern), and the time-of-flight delays until thereflected signals are received are used to generate a 3D representationof the underwater scene. Light signals 197 represent the transmittedsignal beam (where the transmitter and receiver are directly interfacedwith, or located in, the water being examined), and received lightsignals 196 (which are due to light interactions with anomalies orobjects and are shown as water-object-interaction to water-detectorlight signals). In a similar manner, light signals 187 represent thetransmitted signal beam (where the transmitter and receiver are in air(e.g., aboard an aircraft 48) above the seawater surface (water-airinterface 89), and thus propagate through air before going into thewater being examined), and received reflections 186 representwater-object-interaction to in-air-detector light signals (which mayalso include a reflection from the seawater surface from the transmittedbeam that did not enter the water).

FIG. 1D is a block diagram of a visible-light sensing/imaging system 104using a frequency-quadrupled IR laser transmitter system 193 thatoutputs a pulsed waveform (amplitude modulated) transmitted laser beam197 having a narrowband (e.g., in some embodiments, a FWHM (full-widthhalf-maximum) linewidth of 1 nm or less) light wavelength in awavelength range of about 450 nm to about 500 nm (this is transmittedtoward the scene to be imaged) and a receiver-detector system 160 thatdetects light in the same narrowband wavelength range of (this isreceived from reflections from the water surface, underwater objects andsea life, thermoclines and the bottom surface), and processes thereceived reflections 196 (which are water-object-interaction towater-detector light signals) to generate 2D and/or 3D image informationwhich is output and/or stored. In a similar manner, received reflections186 in FIG. 1C represent water-object-interaction to in-air-detectorlight signals (which may also include a reflection from the seawatersurface that must be dealt with (e.g., removed in order to better detectsignals from under the water surface) and/or used as a heightreference). In some embodiments, the receiver-detector system 160includes a narrowband wavelength filter 166 to essentially eliminate allwavelengths other than the wavelength of the narrowband transmittedlight 197, in order to increase the signal-to-noise ratio. In someembodiments, the pulsed waveform that is to be transmitted is suppliedto a transmission modulator 161 which controls a modulation of the laserlight generated by laser 162 (in some embodiments, this includes athulium-doped fiber laser and/or thulium-doped fiber amplifier asdescribed below). In some embodiments, the pulse-modulated and amplifiedintermediate laser beam has a wavelength of approximately 1880 nm (1.88microns) that is coupled to frequency quadrupler 163 (which converts thewavelength to 25% of the original wavelength, which is 470 nm (0.47microns). In some embodiments, this pulsed 470-nm output beam isdirected in a pattern of particular directions (e.g., an X-Y scanpattern) by optional beam-steering unit 164 (e.g., directed tounderwater scene 88). In some embodiments, an optional receiver pointerunit 165 is also synchronized and scanned along the same pattern ofparticular directions (e.g., the same X-Y scan pattern). This beamsteering and reception pointing directs a majority of the output beam atany one time toward a particular angle (the portion of the 3D scene tobe imaged), and restricts the received light to light received from thatsame direction in order to further improve signal strength and thesignal-to-noise ratio of the signal. The portion 196 of the pulsedoutput signal 197 that is reflected and received by the receiver 160(i.e., either in the same vehicle as the transmitter or in a secondvehicle at a distance from the transmitter vehicle) is processed asdescribed in the next paragraph (in the description of the receivingapparatus 160). In some embodiments, the transmitted beam 197 ispolarized. In some embodiments where the transmitting/receiving vehicleis an aircraft 48, beam 197 is linearly polarized in a direction thatenhances transmission through the air-water interface (since light ofone polarization intersecting an air-water interface will reflect, whilelight of the orthogonal polarization will be transmitted through theair-water interface (especially when the angle of intersection matchesBrewster's angle)).

In some embodiments, receiving apparatus 160 is configured to receivelaser-beam reflections of the transmitted beam. In some embodiments,receiving apparatus 160 is in the same vehicle as the transmitter 193,while in other embodiments, receiving apparatus 160 is in a differentvehicle located at a position that better receives the reflections froma particular underwater feature. Some embodiments include areceiver-pointer unit 165 that preferentially receives laser light of aparticular wavelength (e.g., 470 nm) and/or polarization from aparticular direction (e.g., from the direction of reflections of thetransmitted beam) in order to increase its signal-to-noise ratio. Insome embodiments, the received laser signal and any associated lightnoise (e.g., other ambient wavelengths that are not desired) is passedthrough filter 166 (e.g., in some embodiments, a narrowband wavelengthbandpass filter) that passes only the desired wavelengths (e.g., havinga FWHM linewidth of 1 nm or less centered at a wavelength of 470 nm) andrejects other wavelengths. In some embodiments, filter 166 includes apolarizing beamsplitter or similar apparatus that obtains two (or more)signals from different polarizations of the received light, wherein eachpolarized beam is detected by a respective beam detector 167 and theprocessing done by signal-reception image-processing unit 168 includessubtracting the signal from one polarization from the signal of anotherpolarization (e.g., to remove ambient light signals that are in bothpolarizations), or other such signal processing to enhance thesignal-to-noise ratio. In some embodiments, the output image data 169 isprocessed to generate X3D data structures such as can be readily viewedand manipulated using conventionally available virtual-reality renderingsoftware, in order to enhance the visualization and simplify the storageand transmission of the 3D data.

In some embodiments, the transmitted pulsed light beam includes afrequency-quadrupled laser beam having a broad linewidth or twodifferent polarizations, or the transmitted pulsed light beam includestwo or more frequency-quadrupled laser beams, each having a differentwavelength and/or polarization in order that the received reflectedsignal can be detected and analyzed in a manner that takes advantage ofthe wavelength and/or polarization sensitivity of different scattering,absorption, fluorescence, dispersion (detecting a change betweendifferent amounts of normal dispersion wherein the index of refractionof the material for blue wavelengths is higher than the index ofrefraction of the material for red wavelengths such that the bluer part(shorter wavelengths) of the transmitted spectrum travels slower thanthe redder part (longer wavelengths) of the spectrum, which results inthe temporal spectrum of a pulse being distorted with its shorterwavelengths arriving after its longer wavelengths, or detecting a changebetween normal dispersion and anomalous dispersion wherein the redderpart of the transmitted spectrum travels faster than the bluer part ofthe spectrum) or reflection mechanisms. In some embodiments, the pulsetiming of the different transmitted pulsed signals is made eithersynchronous and simultaneous (wherein each pulse from each source issimultaneous with the pulses from the source having other wavelengths orpolarizations), synchronous and non-simultaneous (wherein each pulsefrom each source is alternated with the pulses from the sources havingother wavelengths or polarizations), or even asynchronous with pulses ofother sources.

The direction-limited wavelength-and-polarization filtered light isdetected by beam detector 167, which generates one or more electricalsignals (representative of the various reflected polarizations and/orwavelength signal data), and these one or more electrical signals is, orare, coupled to image processor 168, which then outputs the image data169 (e.g., 2D or 3D images, X3D or VRML data (i.e., data inindustry-standard data formats used for 3D data or virtual-realitymarkup language formats), or other image, anomaly or object-detectiondata). In some embodiments, the beam-steering unit 164 and thereceiver-pointer unit 165 are included in a vehicle-interface unit 194such as shown in FIG. 1C.

In some embodiments, a single set of the IR laser andfrequency-conversion apparatus is used for both underwatercommunications as described and shown in FIG. 1A and FIG. 1B, as well asfor the imaging and object detection as described and shown in FIG. 1Cand FIG. 1D. In some such embodiments, the information beingcommunicated using the elements described in FIG. 1B is encoded onto theranging pulses 197 that are being scanned for image acquisition, such asdescribed in FIG. 1D, in such a manner that only the pulses in oneparticular direction have the data to be communicated to acommunications target in that particular direction, while pulses inother portions of the scan pattern would not include the communicateddata but instead would include noise or other pattern data or no patterndata. For example, if the scan pattern of pulses from the transmitterwere transmitted at angles that formed an X-Y Cartesian pattern of 1024pixels by 1024 pixels, and the vessel to be communicated with werelocated in the direction of a given pixel location (e.g., pixel gridlocation {348, 750}), then the pulses to that pixel grid location aremodulated (e.g., the timing of the pulse, width of the pulse, amplitudeof the pulse, and/or some other attribute of the pulse is varied) basedon the data to be communicated, while pulses to other pixel gridlocations could be varied in other ways in order that the communicateddata and the direction to the target vessel could be disguised.

In some embodiments, the submarine (e.g., vehicle 116 of FIG. 1C)wishing to remain hidden or to disguise its position would include areceiver capable of detecting a scan pattern of the imaging orobject-detection laser beam from another vehicle, e.g., a surface vessel47 or aircraft 48, and to then transmit its own scan pattern withdiffering delays relative to the transmitted pulses in order that thescattering or reflections (or other light interactions) from the beam ofthe submarine 116 would be received by surface vessel 47 or aircraft 48and be misinterpreted as innocuous terrain scattering, reflectionsand/or noise. If the imaging vehicle were transmitting CW or large-areapulsed light to illuminate and image an entire scene, the submarinecould transmit a supplementary illumination toward the sea bottom thatcould hide thermocline disturbances caused by its passing.

FIG. 2A is a block diagram of a visible-output master-oscillatorpower-amplifier (MOPA) system 201 using an IR semiconductor-and-fiberMOPA laser 210A. In some embodiments, system 201 includes anelectronically-switched IR-signal semiconductor-and-fiber MOPA laser210A that includes a rare-earth-doped fiber 224A (e.g., an optical fiberhaving a core doped with thulium (Tm) and/or holmium (Ho)) havingcouplers 219 at its ends that are used receive a free-space input signalbeam 294 from the seed laser 242 in the upper-left of the diagram andamplify that signal to form a free-space intermediate output beam 296.In some embodiments, information is encoded and used by controller 235to control some aspect of the pulse train (e.g., timing of pulses)imposed on input signal beam 294 in order that theamplified-and-wavelength-converted output beam 299 conveys thatinformation to a remote receiver (such as described above in FIG. 1A andFIG. 1B). In other embodiments, the pulse train is controlled (e.g., byregular timing of pulses) in order that the output beam, when reflectedor scattered by underwater anomalies (anomalies such as turbulence on anunderwater thermocline or halocline, or objects (such as a submarine),or topography (such as the underwater seascape)) conveys that underwaterremote information as various delays on the reflected light pulsesreceived by a local receiver (such as described above in FIG. 1C andFIG. 1D). This configuration of system 201 provides additionalamplification by power fiber amplifier 224A outside the seed-lasercavity. In some embodiments, MOPA laser 210A also includes an inputdichroic beam splitter 213A that receives a seed laser signal 294 fromseed laser 242 that has passed through bandpass filter 217 (optionallyincluded in some embodiments, and used to narrow the line width of theseed laser signal), and input dichroic beam splitter 213A also removesunused counter-propagating pump light 284 and directs it to pump dump283. In some embodiments, the light from pump laser 214 is multimode(low-quality) light; however, amplifying fiber 224A is configured as alarge-mode-area large-core single-mode amplifier for the signal pulsesfrom seed laser 242. In some embodiments, seed laser 242 includes asemiconductor laser (wherein the seed-laser pulses are obtained bymodulating the electrical current supplied to the semiconductor laser,or are obtained by an optical modulator that amplitude modulates CWoutput of the semiconductor laser), while in other embodiments, seedlaser 242 includes a Q-switched ring-fiber laser such as described belowin FIG. 2B, for example, or in yet other embodiments, a CW ring-fiberlaser such as described below in FIG. 2B but omitting the Q-switchmodulator and instead providing an optical modulator that amplitudemodulates CW output of the CW ring-fiber laser. In still otherembodiments, a laser diode (i.e., a semiconductor device such as, forexample, SAR-1850-20 laser diode operating at 1850 nm, SAR-2004-DFBlaser diode operating at 2004 nm, SAR-2050-DFB laser diode operating at2050 nm, or SAR-2350-20 laser diode operating at 2350 nm, each availablefrom Sarnoff Corporation of 201 Washington Road, PO BOX 5300, Princeton,N.J., 08540) is either operated in pulsed mode to generate IR seedpulses or operated in continuous-wave mode to generate an initial IRlaser signal that is then modulated using an optical modulator togenerate IR seed pulses. These IR seed pulses are then amplified by arare-earth-doped optical fiber and then frequency quadrupled. In yetother embodiments, a high-power semiconductor laser diode (i.e., asemiconductor device such as, for example, a PEARL laser diode operatingat 1900 nm (or other predetermined wavelength between about 1800 nm andabout 2000 nm) and up to 20 watts or more output power, available fromnLight Corporation of 5408 NE 88th Street, Building E, Vancouver, Wash.98665) is used to generate high-power IR pulses that are frequencyquadrupled for use in the systems of the present invention.

In some embodiments, pump laser 214 launches pump light via an outputdichroic beam splitter 213B in a counter-propagating direction to theamplified signal light that is emerging as beam 296, and bandpass filter221 between lens 231 and beam splitter 213B is used to further narrowthe bandwidth (also called the linewidth) of intermediate output beam296. In some embodiments, output coupler/beam splitter 213B includes adichroic beam splitter that reflects the high-power short-pulse signallight (so that the high-power signal beam does not pass through thiselement) and passes pump light straight through unimpeded into coupler219. In some embodiments, the MOPA-laser output beam 296 is focused bylens 231 to generate beam 297 which is directed intowavelength-conversion device 250 (in some embodiments, device 250includes a first frequency doubler 232 (e.g., that converts1880-nm-wavelength light to 940-nm-wavelength light) and a serial secondfrequency doubler 233 (e.g., that converts 940-nm-wavelength light to470-nm-wavelength light)), and its output 298 is collimated by lens 234into optional beam-steering or beam-aiming device 240 to form directedoutput beam 299. In some embodiments, system 201 fits into a footprintof about 50 cm by 40 cm or smaller.

FIG. 2B is a block diagram of an IR-output system 202 using an IRpower-oscillator ring laser 210B. In some embodiments, system 202outputs a laser signal 297 that is (at least partially) used directly(e.g., for IR air-to-air communications), while in other embodiments,some or all of signal 297 is passed through a wavelength converter 250(which, in some embodiments, includes a first frequency doubler 232 anda serial second frequency doubler 233, or, in other embodiments,includes an optical parametric oscillator (OPO) or optical parametricamplifier (OPA) or other suitable non-linear wavelength-conversiondevice). In some embodiments, electric controller 237 generates a shaped(e.g., in some embodiments, not a square pulse) electrical signalsuitable to drive Q-switch 215 (e.g., an optical modulator that, in someembodiments, is operated in a manner to make the ring-laser system 202be Q-switched, and which in other embodiments, alternatively is operatedto be an amplitude optical modulator of an amplitude modulated butotherwise CW optical signal) in a manner that generates amore-constant-amplitude (e.g., substantially square-wave) output-lightpulse train. In some embodiments, information (e.g., data to becommunicated) is encoded and used by controller 237 to control someaspect of the pulse train (e.g., timing, width, and/or amplitude ofpulses) in order that the output beam 299 conveys that encodedinformation to a remote receiver (such as described above in FIG. 1A andFIG. 1B). In other embodiments, the pulse train is controlled (e.g.,timing of pulses) in order that the output beam, when reflected byunderwater anomalies (such as turbulence on a thermocline) or topography(such as the underwater seascape) conveys that information as areflected light pulse to a local receiver (such as described above inFIG. 1C and FIG. 1D). In the embodiment shown, pump laser 214 has itsoutput coupled into gain fiber 224B using a free-spacewavelength-dependent element such as a dichroic beam splitter 213 thatpasses pump light and reflects signal light (in order to avoid havinghigh-power short-pulse signal pulses pass through the beam splitterwhich could damage the beamsplitter, and instead have the CW orlonger-pulse lower-power pump light pass through the beam-splittingelement). In some embodiments, system 202 has a physical footprint of nomore than 0.2 meters (e.g., 50 cm by 40 cm).

FIG. 3A is a block diagram of a one-or-more-wavelength-output system 301using an IR laser 310A. In some embodiments, system 301 operates in amanner substantially similar to that described for system 202 of FIG.2B, except that the fiber 224B of FIG. 2B, when used as laser 310A, isdoped with a rare-earth (such as Yb) that lases efficiently at a shorterIR wavelength (e.g., such as 1064 nm, which is shorter than the 1800 to2200 nm used, e.g., in the embodiments described above for FIG. 2A andFIG. 2B), and laser 310A outputs an initial high-power intermediatelaser beam 380 having the shorter IR wavelength, which is firstfrequency-down-converted to two longer wavelengths by using thehigh-power shorter IR wavelength laser output beam 380 as the pump-lightinput to optical parametric oscillator (OPO) 340A and one or morefrequency quadruplers 350.1 and 350.2 are placed at the respectiveoutputs 381 and 382 of OPO 340A to generate output beams 384 and 385respectively. In some embodiments, a frequency doubler (e.g., 332.3) isalso used as a later stage for the unconverted pump light 383 of the OPOto generate output beam 386. In this way, wavelength converters operateto frequency-up-convert each of up to three wavelengths that are outputby OPO 340A. In some embodiments, OPO 340A is operated to outputcoherent light at each of up to three different wavelengths, viz., lightbeam 383 at a pump wavelength λ_(o) which is the unconverted portion ofintermediate signal 380 (i.e., the output beam from laser 310A, which invarious embodiments includes a ytterbium (Yb) doped laser lasing at 1064nm, but which is otherwise configured as the IR laser 210A of FIG. 2A,the IR laser 210B of FIG. 2B, the IR laser 610 of FIG. 6, or the IRlaser 710 of FIG. 7A), idler light beam 382 at an idler wavelength λ_(i)that is one of the two wavelengths resonant in OPO 340A, and signallight beam 381 at a signal wavelength λ_(s) that is the other of the twowavelengths resonant in OPO 340A. (Note that the “signal” beam and the“idler” beam and the unconverted “pump” beam are terms of art inrelation to an OPO; as used in these embodiments, any one or two or evenall three output beams derived from the OPO 340A of FIG. 3A and OPO 340Bof FIG. 3B are used as signals.) A more detailed description of an OPOthat can be used in system 301 is provided in U.S. Pat. No. 7,620,077titled “APPARATUS AND METHOD FOR PUMPING AND OPERATING OPTICALPARAMETRIC OSCILLATORS USING DFB FIBER LASERS,” which issued Nov. 17,2009 (which is incorporated herein by reference). In some embodiments,the resonant wavelength of OPO 340A is adjustable by adjusting itsintracavity wavelength bandpass filter 341 (e.g., a Fabry-Perotresonator). For example, if the OPO pump wavelength of light beam 380were 1064 nm, and one of the wavelength-converted output beams (e.g.,signal beam 381) has a wavelength of 1850 nm, the otherwavelength-converted output beam's (e.g., idler beam 382's) wavelengthwould be 2504 nm. If the pump wavelength is then doubled by wavelengthconverter 332.3, its output beam 386 would have a wavelength of 532 nm(green), and the other two beams 381 and 382 would be frequencyquadrupled (to a wavelength of 462.5 nm (blue) of output beam 384 and awavelength of 632 nm (red) of output beam 385, respectively), thusproviding three output colors that could be used to distinguish imagableunderwater objects. Of course, other embodiments use different pumpwavelengths and different resonant wavelengths to obtain differentoutput wavelengths.

FIG. 3B is a block diagram of a one-or-more-wavelength-output system 302using an IR laser 310B. In some embodiments, system 302 uses a frequencyquadrupler 250 between a thulium-doped laser 310B ((in some embodiments,frequency quadrupler 250 is implemented as a pair of serially operatingfrequency doublers such as frequency doublers 232 and 233 of FIG. 2A orFIG. 2B, using the output beam 297 from laser 310B (which in variousembodiments includes the IR laser 210A of FIG. 2A, the IR laser 210B ofFIG. 2B, or any of the other IR laser embodiments described herein))having a wavelength of about 1900 nm (e.g., using a thulium-doped gainfiber in laser 310B) to obtain intermediate beam 299 having a 475-nmwavelength (i.e., corresponding to output beam 299 of FIG. 2B, but inthe embodiment shown in FIG. 3B this beam is further wavelengthconverted to obtain other wavelengths) that is input to an OPO 340Bhaving one resonant wavelength a little longer than 950 nm (e.g., 960nm) and thus another resonant wavelength a little shorter than 950 nm(e.g., 940 nm), and each of these two converted wavelengths (idler andsignal) is frequency doubled to obtain output wavelengths at 470 nm and480 nm, and the residual pump wavelength at 475 nm is also output. Insome embodiments, such closely-spaced wavelengths are used todistinguish objects having different interactions with the differentwavelengths.

In some embodiments of the present invention, the gain fiber(s) used inthe master-oscillator and/or power-amplifier stages of each of thedescribed ring lasers are doped with Tm (in some embodiments, the lasingsignal wavelength is about 1900 nm, while in other embodiments, thelasing signal wavelength is in the range of about 1700 nm to about 2100nm), and in some embodiments, the pump light for the MOPA has awavelength in the range of about 780 nm to 810 nm.

In some embodiments of the present invention, the gain fiber(s) used inthe master-oscillator and/or power-amplifier stages of each of thedescribed ring lasers is doped with Tm (in some embodiments, the lasingsignal wavelength is about 1940 nm, while in other embodiments, thelasing signal wavelength is in the range of about 1880 nm and about 2040nm, and, in some embodiments, a pump wavelength of about 794 nm is used,such as described in co-owned U.S. patent application Ser. No.12/050,937, which is incorporated herein by reference, and which issuedas U.S. Pat. No. 8,202,268 to Wells et al. on Jun. 19, 2012).

In some embodiments, the wavelength filter of the ring lasers can beadjusted by, e.g., tilting the filter element to achieve the desiredwavelength of the ring laser.

In some embodiments of each of the other ring lasers in the figures(e.g., those shown in FIG. 1B, FIG. 1D, FIG. 2A, FIG. 2B, FIG. 6, andFIG. 7A) described herein, the respective gain fibers for the masteroscillators, power oscillators, and/or power amplifiers include passivePM end fibers spliced to one or both ends of the gain fibers, in orderto reduce heat in portions of the fiber that are not in good thermalcontact with a heat sink (such as a water-cooled mandrel as described inU.S. patent application Ser. No. 12/053,551 filed on Mar. 21, 2008,titled “HIGH-POWER, PULSED RING FIBER OSCILLATOR AND METHOD,” which isincorporated herein by reference, and which issued as U.S. Pat. No.7,876,803 on Jan. 25, 2011).

For example, some embodiments use a rare-earth-doped fiber (e.g., insome embodiments, the length of gain fiber is in the range of 1-5 meterslong; in some embodiments, having a thulium-doped core) having a corediameter (e.g., in some embodiments, a constant core diameter for thelength of the fiber) in a range of about 10 microns to about 25 micronsor larger and an outer diameter of between about 250 microns and about400 microns (fibers such as these are available or can be ordered fromcompanies such as Nufern, 7 Airport Park Road, East Granby, Conn. 06026,Coractive, 2700 Jean-Perrin, Suite 121, Quebec (Qc), Canada, G2C 1S9, orOFS, 2000 Northeast Expressway, Norcross, Ga., 30071).

As described in co-owned U.S. patent application Ser. No. 12/050,937titled “METHOD AND MULTIPLE-MODE DEVICE FOR HIGH-POWER SHORT-PULSE-LASERABLATION AND CW CAUTERIZATION OF BODILY TISSUES” that was filed Mar. 18,2008 (which is incorporated herein by reference, and which issued asU.S. Pat. No. 8,202,268 on Jun. 19, 2012), some embodiments can alsoprovide a continuous wave (CW) mode or quasi-CW mode by outputting aCW-activation signal in order to enable CW operation of the masteroscillator without Q-switching.

In some embodiments, a power-amplifier pump laser is used to pump thepower-amplifier stage (in some embodiments, this power-amplifier pumplaser is a semiconductor laser bar (e.g., up to 50 watts or more in someembodiments)) that generates pump light having a wavelength ofapproximately 785 nm, which is effectively absorbed by the gain fiber inorder to amplify the signal light to form the output signal beam. Insome embodiments a master-oscillator pump laser is a semiconductor laserbar that generates pump light (e.g., up to 25 watts or more in someembodiments) also having a wavelength of approximately 785 nm, which iseffectively absorbed by the master-oscillator's gain fiber in order toamplify feedback signal light to form the intermediate output (seed)signal beam.

FIG. 4 is a graph 401 showing the spectrum of sunlight. The features inthe spectrum labeled with various letters correspond to “Fraunhoferlines.” At the wavelengths corresponding to Fraunhofer lines, the solarbackground is reduced, so the signal/background ratios for communicationor LIDAR systems may be improved by operating at one of thesewavelengths. In some embodiments, the wavelength of the quadrupled lightbeam is set to the wavelength of the feature F (one of the Fraunhoferfeatures (absorption lines in the solar spectrum)) shown in graph 401.In some embodiments, the wavelength of the quadrupled light beam is setto the wavelength of one of the other Fraunhofer features (absorptionlines in the solar spectrum).

FIG. 5A is a series of graphs 501 that include a graph 502.1 showingsunlight penetration in seawater at 1 meter, a graph 502.10 for 10meters, and a graph 502.100 for 100 meters.

FIG. 5B is a graph 503 showing polarization differences in lighttransmission through water.

FIG. 5C is a graph 504 showing polarization differences in lighttransmission through water.

FIG. 6 is a block diagram of a visible-output system 601 using an IRring laser 610. In contrast to the power-oscillator operation of system201 of FIG. 2A, the ring laser of FIG. 6 (and other MOPA lasers asdescribed in U.S. patent application Ser. No. 12/053,551 filed on Mar.21, 2008, titled “HIGH-POWER, PULSED RING FIBER OSCILLATOR AND METHOD,”which is incorporated herein by reference (and which issued as U.S. Pat.No. 7,876,803 on Jan. 25, 2011), operates in a master-oscillatorpower-amplifier (MOPA) mode. In some embodiments, system 601 includes aQ-switched IR-signal fiber ring laser 610 that includes arare-earth-doped (e.g., in some embodiments, thulium-doped) opticalfiber 624 having fiber-to-free-space couplers (formed by fused silicaendcap 623 and lens 622 (at the lower end in FIG. 6) and fused silicaendcap 611 and lens 612 (at the upper end in FIG. 6)) that are used toform a free-space in-cavity beam 604. In the embodiment shown, ringlaser 610 operates in a low-power master-oscillator mode, wherein theoscillator and power-amplifier functions of the laser are separated, andpower amplifier 620, external to the oscillator ring, generates thehigh-power pulse 698 from the output signal 697 of the lower-power ringlaser 610.

In some embodiments, ring laser 610 also includes a pump laser 603(e.g., a moderately low-power diode laser that outputs a continuous-wave(CW) signal during operation of the laser), a dichroic beam splitter 613(also labeled M1, indicating mirror 1, which reflects only the pumplight) that passes the signal wavelength (traveling in a clockwisedirection) but reflects the pump light in a counter-propagatingcounter-clockwise direction into the ring fiber 624, a first polarizingbeam splitter 614, Q-switch modulator 615 driven by a pulsed drivingvoltage from pulse source 605, a second polarizing beam splitter 616,optical isolator 619 used to obtain unidirectional (in a clockwisedirection, in the embodiment shown) signal in the ring laser, firsthalf-wave plate 617, a third polarizing beam splitter 618 used to outputthe infrared intermediate output beam 697 through bandpass filter 655and then dichroic beamsplitter 625 (also labeled mirror M2, which, insome embodiments, is replaced by an optical isolator that prevents anyhigh-power signal or pump light from traveling in a backward directionfrom the power amplifier stage 620 into the master oscillator 610),wherein the signal 697 is amplified by power amplifier 620 to formhigh-power output signal pulses 698 that exit through lens 631. In someembodiments, low-power infrared intermediate output beam 697 passesthrough a second bandpass filter 655 before entering the power amplifier620. In some embodiments the wavelength spectrum of low-power infraredintermediate output beam 697 is narrowed and determined by bandpassfilter 655 (e.g., in some embodiments, a filter having a1.0-1.5-nm-linewidth passband; in other embodiments, the filter has aless-than-1-nm pass-band). In the master oscillator ring 610, theclockwise-traveling signal light continues from polarizing beam splitter(PBS) 618 through bandpass filter 621 (e.g., a Fabry-Perotinterferometer having an angle-adjustable wavelength selectivity). Insome embodiments, the wavelength spectrum of beam 604 is narrowed anddetermined by bandpass filter 621 (e.g., in some embodiments, a filterhaving a 0.7-nm-linewidth passband, such as a bandpass filter partavailable from Barr Associates, Inc., 2 Lyberty Way, Westford, Mass.01886 USA, having a web address www.barrassociates.com). In someembodiments, the pump light from pump laser 603 counter-propagatesrelative to beam 604, and is launched or combined (in thecounter-clockwise direction in the figure) into the ring beam 604 bydichroic beam splitter 613 (e.g., in some embodiments, a dichroic beamsplitter mirror, such as is available from Barr Associates, Inc.).

In some embodiments, isolator 619 (e.g., in some embodiments, an opticalisolator such as is available from Electro-Optics Technology, Inc., 5835Shugart Lane, Traverse City, Mich. 49684 USA having a web addresswww.eotech.com) ensures unidirectional (clockwise direction in thefigure) lasing in the ring laser 610. In some embodiments, the gainfiber 624 is thulium doped to lase at about 1880 nm (e.g., in someembodiments, about 2 meters of thulium-doped gain fiber). In someembodiments, the Q-switch modulator 615 is an RTP Pockels cell (e.g., insome embodiments, Q-Switch: RTP, 4×4×20 mm from Raicol Crystals Ltd., 15Giron St., Industrial Zone, Yehud, 56217 Israel, with a web site atwww.raicol.com (as described in detail previously for FIG. 2A)).

In some embodiments, the half-wave retardation plates 617 and 646 (e.g.,in some embodiments, part number CWO-1960-02-04 available from LatticeElectro-Optics of Fullerton, Calif. www.latticeoptics.com; or half-waveplates such as are available from CVI Laser, L.L.C., 200 Dorado SE,Albuquerque, N.Mex. 87123 USA, having a web address www.cvilaser.com)each provide a rotation in the direction of polarization of beam 604 byan amount greater than 45 degrees and less than 90 degrees, in someembodiments, with an angle set or optimized by measuring the outputpower with a power meter and maximizing the output power by adjustingthe polarization angle. In some embodiments, beam splitters 614, 616,and 618 are polarizing beam-splitting (PBS) cubes (e.g., in someembodiments, such as are available from CVI Laser, L.L.C., or aGlan-Thompson (“walk-off”) polarizer). In some embodiments, PBS 614cleans up the polarization of the signal light before it entersmodulator 615 (e.g., an electro-optic RTP Q-switch), while the secondPBS 616 is used with the modulator 615 to pass or block the signal beam604. Half-wave plate 617 is set to an angle that rotates the directionof polarization such that most of the signal beam is output to low-powerinfrared intermediate beam 697, while passing a small portion throughnarrow-linewidth bandpass filter 621 to seed further lasing. In someembodiments, a second half-wave plate 646 is used to rotate the seedlight polarization to match the polarization angle of the gain fiber624. In some embodiments, gain fiber 624 is apolarizing/polarization-maintaining amplifying fiber, e.g., in someembodiments, one that includes two “stress rods,” one on either side ofits 25-micron core in a 250-micron-diameter fiber, to promote andmaintain polarized amplification in the fiber. In some embodiments, thebandpass filter 621 is a 0.7-nm-linewidth thin-film interferometer setat angle to select the desired wavelength. Lens 622 focuses the seedlight into the endcap 623 of fiber 624.

In some embodiments, the low-power pulsed infrared intermediate beam 697is focused by lens 626 into an endcap of power-amplifier fiber 627 whereit is amplified using pump light from power-amplifier pump-lasersubsystem 640 (which, in some embodiments, includes a plurality of laserdiodes 648 that are directed into respective fibers that are joined by afiber coupler into a single fiber, and output through afiber-to-free-space coupler (e.g., in some embodiments, formed by afused silica endcap) and collimated by lens 641 into a parallel beamthat is reflected by dichroic beamsplitter 629 to lens 628 that focussesthe pump beam in a counter-propagating direction (right-to-left in thediagram) to enter through a free-space-to-fiber coupler into fiber 627.Amplified signal pulses from the fiber (propagating in a left-to-rightdirection in FIG. 6) are collimated by lens 628 and pass throughdichroic beamsplitter 629 to output as pulse beam 698, which is focusedby lens 631 into non-linear device 650 (e.g., a series of twoconventional frequency-doubling devices that change the outputwavelength to one quarter its starting wavelength (e.g., in someembodiments, from 1880 nm to 470 nm)), as described above for FIG. 2A,and the frequency-quadrupled pulses are collimated by lens 633 andoutput as pulsed beam 699.

In some embodiments, the high-power pulsed infrared intermediate beam698 is frequency quadrupled from 1880 nm (IR) to 470 nm (blue) by awavelength-conversion device 650 that includes a first frequency-doublernon-linear crystal such as periodically poled MgO-doped lithium niobate(PPMgOLN) or periodically poled MgO-doped lithium tantalate (PPMgOLT)(used for medium to high power embodiments, because the MgO dopingincreases the optical-damage threshold) or periodically poled lithiumniobate (PPLN) (used for low-power embodiments) that converts the 1880nm (IR) to 940 nm (IR) wavelengths and a second frequency-doublernon-linear crystal such as lithium borate (LBO), or periodically polednonlinear frequency doubling crystal such as PPMgOLN, PPMgOLT, PPZnOLN,PPZnOLT, stoichiometric PPLN (called PPSLN herein), or stoichiometricPPLT (called PPSLT herein) that converts the 940 nm (IR) to 470 nm(blue) wavelengths. In some embodiments, the first and secondfrequency-doubler non-linear crystals are selected from among those(such as PPMgOLN, PPMgOLT, PPZnOLN, PPZnOLT, PPSLN, or PPSLT) describedin U.S. Patent Application Publication 2007/0253453 titled “Solid-statelaser arrays using” and U.S. Patent Application Publication 2008/0317072titled “Compact solid-state laser,” which are incorporated herein byreference. In some such embodiments, the first frequency-doublernon-linear crystal is different than the second frequency-doublernon-linear crystal. Other embodiments use PPKTP (periodically poledpotassium titanyl phosphate) or PPSLT that are quasi-phasematched. Inother embodiments, lithium borate (LBO used in a non-criticallyphasematched configuration and operated at rather a high temperature ofabout 280 degrees C. for generating wavelengths about 485 nm) or bismuthborate (BiBO used in a critically phasematched configuration) are usedfor the frequency-quadrupling operation.

In some embodiments, the blue output beam is used for the underwatercommunications, imaging, or LIDAR applications shown in FIG. 1A, FIG.1B, FIG. 2A, and/or FIG. 2B; in other embodiments, the blue-light outputbeam(s) is (are) used for any other suitable purpose, such as variousmedical purposes. In some embodiments, intermediate beam 698 includes upto 5000-watt pulses (up to 10 watts continuous) or more, and up to 25percent or more of the intermediate beam 698 is converted to blue lightof output beam 699, resulting in blue-light output of up to 1250 wattspeak (up to 2.5 watts continuous) or more.

In some embodiments, the ring-laser output beam 698 is focused by lens631 into wavelength-conversion device 632 (in some embodiments, afrequency quadrupler that quadruples the frequency, and thus quartersthe wavelength of the light from infrared at 1880 nm to blue at 470 nm),and its output is collimated by lens 633 to form output beam 699. Insome embodiments, a wavelength-selective dichroic mirror (not shown) isused in the output beam 699 to pass the converted wavelengths and blockany residual infrared wavelengths.

In some embodiments, the high-power pulsed infrared intermediate beam698 is frequency quadrupled from 1880 nm (IR) to 470 nm (blue) asdescribed above for FIG. 2A, using blue module 630. In some embodiments,the blue output beam is used for the underwater communications, imaging,or LIDAR applications shown in FIG. 1A, FIG. 1B, FIG. 2A, and/or FIG.2B; in other embodiments, the blue-light output beam(s) is (are) usedfor any other suitable purpose, such as possible medical purposes.

In some embodiments, intermediate beam 698 includes up to 5000-wattpulses (up to 10 watts continuous) or more, and up to 50 percent or moreof the intermediate beam 898 is converted to blue light, resulting inblue-light output of up to 2500 watts peak (up to 5 watts continuous) ormore.

In some embodiments, blue module 630 includes one or more dichroicmirrors (not shown) on output beam 699 that are used to removeunconverted IR pump light from power amplifier 620, and/or to add anoptional auxiliary signal beam that is inserted to be co-axial with theoutput beam 699 (e.g., in some embodiments the auxiliary signal beam isgenerated by a low-power continuous-wave semiconductor diode laser witha wavelength that is substantially different from the wavelength ofconverted blue pulsed beam 699) such that output beam 699 includes theblue pulsed beam co-axially aligned with the auxiliary signal beam (suchas described in U.S. patent application Ser. No. 12/053,551 filed onMar. 21, 2008, titled “HIGH-POWER, PULSED RING FIBER OSCILLATOR ANDMETHOD,” which is incorporated herein by reference, and which issued asU.S. Pat. No. 7,876,803 on Jan. 25, 2011).

FIG. 7A is a block diagram of a multiple-wavelength-output Q-switchedring-laser system 701 using an IR ring laser 710. In some embodiments,ring laser 710 includes a rare-earth-doped gain fiber 724, a Q-switch712 controlled by electronic controller 730, and an output coupler 711(e.g., a polarizing beam splitter in some embodiments) that outputs anintermediate output beam 713 having a wavelength of λ_(o) that isprocessed by wavelength-conversion device 720 (in some embodiments, anon-linear optical device, such as a wavelength doubler or OPO or othersuitable device) that outputs one or more output wavelengths 721 . . .722 (e.g., wavelengths λ₁ through λ_(N)).

FIG. 7B is a graph 703 of the amplitude (in voltage, current or othersuitable unit) over time of an electrical-pulse waveform 741. In someembodiments, an electrical pulse having an amplitude that increases overtime (a ramp shape, such as shown) is output from controller 730 of FIG.7A and used to drive Q-switch 712, where the increasing amplitude of theelectrical pulse gradually opens the Q-switch in a manner thatcompensates for the decrease in cavity gain (and/or external amplifiergain) over time.

FIG. 7C is a graph 704 of an idealized optical-pulse waveform 742(representing the shape of pulses at output 713 of the above figures,wherein the pulse shape is approximately “square” (substantiallyconstant over the duration of the pulse) because of the compensatingnature of the Q-switch driving voltage.

FIG. 8 is a block diagram of a materials-processing system 800 using oneor more of the ring-laser systems 811 as described herein (e.g.,ring-laser system 811 can include system 102 of FIG. 1B, system 104 ofFIG. 1D, system 201 of FIG. 2A, system 202 of FIG. 2B, system 301 ofFIG. 3A, system 302 of FIG. 3B, system 601 of FIG. 6 and/or system 701of FIG. 7A). In some embodiments, materials-processing system 800includes a production unit 810 that is controlled by one or morecontrollers 812 and which uses the laser output of one or morering-laser systems 811. In some embodiments, each ring-laser system 811includes one or more of the designs exemplified by the systems describedabove and shown in FIG. 1B, FIG. 1D, FIG. 2A, FIG. 2B, FIG. 3A, FIG. 3B,FIG. 6, and FIG. 7A. In various embodiments, the light output from theIR laser (e.g., light from beam 297 of FIG. 2B) is also used directly(in addition to the blue wavelength-converted light 299 of FIG. 2B),while in other embodiments, the light output from the IR laser is onlyan intermediate beam that is further wavelength converted by any of thewavelength-conversion devices described herein (such as a wavelengthdoubler or tripler, optical parametric generator (OPG), opticalparametric oscillator (OPO), or operational parametric amplifier (OPA)or the like) and then the wavelength-converted light is output.

In some embodiments, the present invention provides high-power blueoutput pulses that can be used to remove paint, machine via holes (smallholes in electronic substrates or printed circuit boards (PCBs)), metaland/or semiconductor annealing, laser welding, semiconductor-memoryrepair (e.g., opening metal lines to connect and/or disconnect sparesections of memory for other sections that have errors, thus increasingthe yield of usable chips in memory manufacture), laser trimming ofprecision resistors (e.g., for analog-to-digital converters anddigital-to-analog converters), other materials processing and/or thelike.

Some embodiments include a materials-processing system having one ormore of the laser systems described herein that is used to provide thelaser energy for the materials-processing operation.

In some embodiments, the present invention provides a method thatincludes optically pumping a fiber ring laser having a beam path;forming a first signal beam in the beam path of the fiber ring laser;Q-switching the first signal beam; and extracting an intermediate outputbeam from the first beam. In some embodiments, this first signal beam isa first free-space signal beam. In some embodiments, the intermediateoutput beam is frequency quadrupled to generate a frequency-quadrupledbeam.

In some embodiments, the intermediate output beam any of the embodimentsof the present invention described herein has a wavelength in a range of1800 nm to 2000 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 450 nm to 500 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1801 nm to 1820 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 450.25 nm to 455 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1821 nm to 1840 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 455.25 nm to 460 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1841 nm to 1860 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 460.25 nm to 465 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1861 nm to 1880 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 465.25 nm to 470 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1870 nm to 1890 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 467.5 nm to 472.5 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1881 nm to 1900 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 470.25 nm to 475 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1901 nm to 1920 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 475.25 nm to 480 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1921 nm to 1940 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 480.25 nm to 485 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1941 nm to 1960 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 485.25 nm to 490 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1961 nm to 1980 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 490.25 nm to 495 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of1981 nm to 2000 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 495.25 nm to 500 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of2001 nm to 2040 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 500.25 nm to 510 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of2041 nm to 2080 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 510.25 nm to 520 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of2081 nm to 2120 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 520.25 nm to 530 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of2121 nm to 2160 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 530.25 nm to 540 nm inclusive. In someembodiments, the intermediate output beam has a wavelength in a range of2161 nm to 2200 nm inclusive and the frequency-quadrupled beam has awavelength in a range of 540.25 nm to 550 nm inclusive.

In some embodiments, the intermediate output beam has a wavelength in arange of 1600 nm to 1800 nm inclusive and the frequency-quadrupled beamhas a wavelength in a range of 400 nm to 450 nm inclusive. In someembodiments, the intermediate output beam has a wavelength longer than2200 nm and the frequency-quadrupled beam has a wavelength longer than550 nm.

In some embodiments, the frequency-quadrupled beam is pulsed, and thepulses are modulated (e.g., via varying the timing and/or width of thepulses) with encoded data, and the encoded beam is transmitted throughwater to communicate the data between two platforms such as anunderwater vessel and another vehicle. In some embodiments, thefrequency-quadrupled beam is focussed and directed in a particulardirection in order to increase the signal and/or the signal-to-noiseratio at the receiving destination platform.

Some embodiments further include opto-isolating the signal beam totravel only in a single direction around the ring.

Some embodiments further include filtering the signal beam to limit alinewidth of the signal beam in the ring.

In some embodiments, the Q-switching is electronically controlled. Insome such embodiments, a timing of a pulse is determined by theelectronically controlled Q-switcher. In some embodiments, theQ-switching includes using a Pockels cell. In some embodiments, theQ-switching includes using a rubidium titanyl phosphate (RTP) Pockelscell.

Some embodiments further include polarizing the signal beam to a linearpolarization on each of two sides of the RTP Pockels cell.

In some embodiments, the extracting of the intermediate output beam fromthe free-space beam includes using a polarizing beam splitter.

Some embodiments further include rotating a direction of polarization byan empirically-determined amount on both of two sides of the outputpolarizing beam splitter.

In some embodiments, the opto-isolating of the signal beam is donebetween the two rotatings of the direction of polarization.

Some embodiments further include wavelength converting the intermediateoutput beam to a wavelength different from a wavelength of theintermediate output beam.

Some embodiments further include opto-isolating the signal beam totravel only in a single direction around the ring; filtering the signalbeam to limit a linewidth of the signal beam in the ring laser, whereinthe Q-switching includes using a rubidium titanyl phosphate (RTP)Pockels cell; polarizing the signal beam to a linear polarization oneach of two sides of the RTP Pockels cell, wherein the extracting of theintermediate output beam from the free-space beam includes using apolarizing beam splitter; rotating a direction of polarization by 90degrees on both of two sides of the polarizing beam splitter, whereinthe opto-isolating of the signal beam is done between the two rotatingsof the direction of polarization; and wavelength converting theintermediate output beam to a wavelength different from a wavelength ofthe intermediate output beam.

In some embodiments, the present invention provides an apparatus thatincludes a fiber-ring laser having a signal beam path, the fiber ringlaser including an optically-pumped polarization-maintaining (PM) gainfiber that forms a portion of the signal beam path; a pump portconfigured to guide pump light into the gain fiber; fiber-end optics ateach of two ends of the gain fiber, the fiber-end optics forming afree-space portion of the signal beam path; a Q-switch in the free-spacesignal beam path; and extraction optics configured to obtain anintermediate output beam from the free-space beam.

Some embodiments further include an opto-isolator in the signal beampath configured to limit the signal beam to travel only in a singledirection around the ring.

Some embodiments further include a wavelength filter in the free-spaceportion of the signal beam path configured to limit a linewidth of thesignal beam in the ring.

In some embodiments, the Q-switch includes a rubidium titanyl phosphate(RTP) Pockels cell.

Some embodiments further include two polarizers in the free-spaceportion of the signal beam path on each of two sides of the RTP Pockelscell to linearly polarize the signal beam on the two sides of the RTPPockels cell.

In some embodiments, the extraction optics include a polarizing beamsplitter.

Some embodiments further include a half-wave plate in the free-spaceportion of the signal beam path on each of two sides of the polarizingbeam splitter.

In some embodiments, the opto-isolator is located between the twohalf-wave plates.

Some embodiments further include a wavelength-converting deviceoptically coupled to receive the intermediate output beam and to converta wavelength of the intermediate output beam to a wavelength differentfrom the wavelength of the intermediate output beam.

In some embodiments, the present invention provides an apparatus thatincludes means for optically pumping a fiber ring laser having a beampath; means for forming a free-space signal beam in the beam path of thefiber ring laser; means for Q-switching the free-space signal beam; andmeans for extracting an intermediate output beam from the free-spacebeam.

Some embodiments further include means for opto-isolating the signalbeam to travel only in a single direction around the ring.

Some embodiments further include means for filtering the signal beam tolimit a linewidth of the signal beam in the ring.

In some embodiments, the means for Q-switching includes a rubidiumtitanyl phosphate (RTP) Pockels cell.

Some embodiments further include means for polarizing the signal beam toa linear polarization on each of two sides of the RTP Pockels cell.

In some embodiments, the means for extracting of the intermediate outputbeam from the free-space beam includes a polarizing beam splitter.

Some embodiments further include means for rotating a direction ofpolarization by 90 degrees on both of two sides of the polarizing beamsplitter.

In some embodiments, the means for opto-isolating of the signal beam islocated between the two rotators of the direction of polarization.

Some embodiments further include means for wavelength converting theintermediate output beam to a wavelength different from a wavelength ofthe intermediate output beam.

Some embodiments further include means for opto-isolating the signalbeam to travel only in a single direction around the ring; means forfiltering the signal beam to limit a linewidth of the signal beam in thering, wherein the Q-switching includes using an optical amplitudemodulator; means for polarizing the signal beam to a linearpolarization, wherein the means for extracting of the intermediateoutput beam from the free-space beam includes a polarizing beamsplitter; means for rotating a direction of polarization on both of twosides of the polarizing beam splitter, wherein means for theopto-isolating of the signal beam is between the two rotatings of thedirection of polarization; and means for wavelength converting theintermediate output beam to a wavelength different from a wavelength ofthe intermediate output beam.

In some embodiments, one or more of the gain fiber(s) of each embodimentincludes a photonic-crystal fiber (PCF) or photonic-crystal rod (PCR).In some such embodiments, the PCF or PCR is polarization maintaining(PM). In some embodiments, one or more of the gain fiber(s) of eachembodiment includes a large-mode-area (LMA) fiber (e.g., in someembodiments, the mode-field diameter in the fiber is larger than about12 microns, while in other embodiments, the mode-field diameter in thefiber is larger than about 25 microns, the mode-field diameter in thefiber is larger than about 50 microns, the mode-field diameter in thefiber is larger than about 75 microns, or the mode field diameter in thefiber is larger than about 100 microns). In some such embodiments, theLMA fiber is polarization maintaining (PM). In some embodiments, the LMAfiber has a numerical aperture (NA) of no more than about 0.15, while inother embodiments, the LMA fiber has an NA of no more than about 0.12,the LMA fiber has an NA of no more than about 0.10, the LMA fiber has anNA of no more than about 0.08, or the LMA fiber has an NA of no morethan about 0.06.

In some embodiments, the amplified IR output beams (either from a poweroscillator or from a MOPA) include pulses of at least 5 kW and anaverage power of at least 10 W. Some embodiments use a plurality of gainstages, which, in some embodiments, are each separated by an isolator (aone-way optical element to prevent backward-traveling light) and/or anarrowband filter (to reduce amplifier spontaneous emission (ASE) and/orclean up the signal pulses). In some embodiments, one or more of thepower amplifier stages use a photonic-crystal fiber (PCF) orphotonic-crystal rod (PCR) as described in U.S. Pat. No. 7,391,561titled “FIBER- OR ROD-BASED OPTICAL SOURCE FEATURING A LARGE-CORE,RARE-EARTH-DOPED PHOTONIC-CRYSTAL DEVICE FOR GENERATION OF HIGH-POWERPULSED RADIATION AND METHOD,” which issued Jun. 24, 2008 and which isincorporated herein in its entirety by reference. In some embodiments,one or more of the power amplifier stages use a non-photonic-crystalfiber having a large mode area (LMA) that has a mode-field diameter ofat least about 12 microns, while other embodiments use an LMA fiber thathas a mode-field diameter of at least about 20 microns, an LMA fiberthat has a mode-field diameter of at least about 40 microns, an LMAfiber that has a mode-field diameter of at least about 60 microns, anLMA fiber that has a mode-field diameter of at least about 80 microns,or an LMA fiber that has a mode-field diameter of at least about 100microns. In some embodiments, the amplified IR output beams includepulses of at least 1 kW. In other embodiments, the amplified IR outputbeams include pulses of at least 2.5 kW. In other embodiments, theamplified IR output beams include pulses of at least 5 kW. In otherembodiments, the amplified IR output beams include pulses of at least 10kW. In other embodiments, the amplified IR output beams include pulsesof at least 25 kW. In other embodiments, the amplified IR output beamsinclude pulses of at least 50 kW. In other embodiments, the amplified IRoutput beams include pulses of at least 100 kW. In other embodiments,the amplified IR output beams include pulses of at least 250 kW. Inother embodiments, the amplified IR output beams include pulses of atleast 500 kW. In other embodiments, the amplified IR output beamsinclude pulses of at least 1000 kW. In other embodiments, the amplifiedIR output beams include pulses of at least 10 kW and an average power ofat least 20 W. In some of each of these embodiments, the amplified IRoutput beam is polarized (in some embodiments, this polarization makesthe non-linear conversion to other wavelengths more efficient).

In some embodiments, the wavelength-conversion device includes apparatusand methods such as described in U.S. Pat. No. 7,471,705 titled“ULTRAVIOLET LASER SYSTEM AND METHOD HAVING WAVELENGTH IN THE 200-NMRANGE,” which issued Dec. 30, 2008 and which is incorporated herein inits entirety by reference. In some embodiments, the wavelength-convertedbeams include output pulses of at least 2.5 kW and an average power ofat least 5 W. In other embodiments, the wavelength-converted beamsinclude output pulses of at least 1.25 kW and an average power of atleast 2.5 W. In other embodiments, the wavelength-converted beamsinclude output pulses of at least 5 kW and an average power of at least10 W. In other embodiments, the wavelength-converted beams includeoutput pulses of at least 25 kW. In other embodiments, thewavelength-converted beams include output pulses of at least 50 kW. Inother embodiments, the wavelength-converted beams include output pulsesof at least 100 kW. In other embodiments, the wavelength-converted beamsinclude output pulses of at least 250 kW. In other embodiments, thewavelength-converted beams include output pulses of at least 500 kW. Insome of each of the above embodiments in this paragraph, thewavelength-converted beams include wavelengths that are one-quarter thewavelength of the amplified IR output beams (e.g., awavelength-converted wavelength of 470 nm if the IR wavelength is 1880nm, or other suitable IR wavelength that is four times the desiredwavelength-converted output wavelength). In some of each of the aboveembodiments in this paragraph, the wavelength-converted beams includewavelengths that are one-third the wavelength of the amplified IR outputbeams (e.g., a wavelength-converted wavelength of about 475 nm if the IRwavelength is 1425 nm, or other suitable IR wavelength that is threetimes the desired wavelength-converted output wavelength; wherein insome embodiments, the 1425-nm IR laser beam (or other suitable IRwavelength) is generated using a cascaded Raman fiber laser). In some ofeach of the above embodiments in this paragraph, thewavelength-converted beams include wavelengths that are one-quarter thewavelength of the amplified IR output beams (e.g., awavelength-converted wavelength of 266 nm if the IR wavelength is 1064nm). In some of each of the above embodiments in this paragraph, thewavelength-converted beams include wavelengths that are one-fifth thewavelength of the amplified IR output beams (e.g., awavelength-converted wavelength of about 400 nm if the IR wavelength is2000 nm, or 475 nm if the IR wavelength is 2375 nm, or other suitable IRwavelength that is five times the desired wavelength-converted outputwavelength). In some of each of the above embodiments in this paragraph,the wavelength-converted beams include wavelengths that are one-sixththe wavelength of the amplified IR output beams (e.g., awavelength-converted wavelength of about 400 nm if the IR wavelength is2400 nm, or 475 nm if the IR wavelength is 2850 nm, or other suitable IRwavelength that is six times the desired wavelength-converted outputwavelength).

In some embodiments, one or more of the gain fiber(s) of each embodimentincludes a single-mode fiber (SMF) or a multi-mode fiber (MMF).

In some embodiments, the present invention provides a method thatincludes providing a fiber gain medium; configuring an optical signalpath that extends through the fiber gain medium such that the opticalpath forms a ring laser having a signal beam; optically pumping thefiber gain medium; forming a free-space signal beam in the opticalsignal path of the ring laser; Q-switching the free-space signal beamoutside the fiber; and extracting, from the free-space signal beam, anintermediate optical signal output beam having a first wavelength.

Some embodiments further include forcing a majority of the signal beamto travel in a first direction around the ring laser.

Some embodiments further include wavelength filtering the signal beam tolimit a linewidth of the signal beam in the ring laser.

In some embodiments, the Q-switching includes polarizing the signalbeam, rotating an angle of polarization of the polarized signal beam,and again polarizing the polarization-rotated signal beam.

Some embodiments further include preferentially amplifying signal lighthaving a first linear polarization direction in the fiber gain medium.

In some embodiments, the extracting of the intermediate optical signaloutput beam from the free-space signal beam includes using a polarizingbeam splitter to split the free-space signal beam into the intermediateoptical signal output beam and a ring-feedback signal beam.

Some embodiments further include rotating a direction of polarization ofthe signal beam by a non-zero amount on both of two sides of thepolarizing beam splitter, wherein the non-zero amount determinesproportions of the intermediate optical signal output beam and thering-feedback signal beam.

In some embodiments, the forcing of the majority of the signal beam totravel in a first direction is performed between the two rotatings ofthe direction of polarization.

Some embodiments further include frequency doubling the intermediateoptical signal output beam to form a second signal output beam having asecond wavelength that is one-half of the first wavelength of theintermediate optical signal output beam.

Some embodiments further include forcing the signal beam to travel in afirst direction around the ring laser; filtering the signal beam tolimit a linewidth of the signal beam in the ring laser, wherein theQ-switching includes using a rubidium titanyl phosphate (RTP) Pockelscell; polarizing the signal beam to a linear polarization on each of twosides of the RTP Pockels cell, wherein the extracting of theintermediate optical signal output beam from the free-space signal beamincludes using a polarizing beam splitter; rotating a direction ofpolarization by 90 degrees on both of two sides of the polarizing beamsplitter, wherein the forcing of a majority of the signal beam is donebetween the two rotatings of the direction of polarization; andwavelength converting the intermediate optical signal output beam to awavelength different from the first wavelength of the intermediateoptical signal output beam.

Some embodiments further include forcing the signal beam to travel in afirst direction around the ring laser; forming a free-space signal beamin the ring laser; filtering the signal beam to limit a linewidth of thesignal beam in the ring laser; amplitude-modulating the signal beam toform pulses; polarizing the signal beam to a linear polarization;extracting the signal beam as an intermediate optical signal output beamfrom the free-space signal beam includes using a polarizing beamsplitter; rotating a direction of polarization on both of two sides ofthe polarizing beam splitter, wherein the forcing of a majority of thesignal beam is done between the two rotating s of the direction ofpolarization; and wavelength converting the intermediate optical signaloutput beam to a wavelength different from the first wavelength of theintermediate optical signal output beam.

In some embodiments, the present invention provides an apparatus thatincludes a ring laser that has an optical signal ring path and furtherhas a signal beam that propagates in the ring laser, the ring laserincluding an optically-pumped gain fiber that forms a first portion ofthe optical signal ring path; a pump port configured to guide pump lightinto the gain fiber; fiber-end optics at a first end of the gain fiberand fiber-end optics at a second end of the gain fiber, the first andsecond fiber-end optics configured to form a free-space second portionof the optical signal ring path between the first end and the second endof the gain fiber such that a free-space signal beam propagates in thefree-space second portion of the optical signal ring path; a Q-switch inthe free-space second portion of the optical signal ring path; andextraction optics configured to obtain an intermediate output beam fromthe free-space signal beam.

Some embodiments further include a first optical component in thefree-space second portion of the optical signal ring path configured toforce a majority of the free-space signal beam to travel in a firstdirection around the ring laser.

Some embodiments further include a wavelength filter located in thefree-space second portion of the optical signal ring path configured tolimit a linewidth of the signal beam in the ring laser.

In some embodiments, the Q-switch includes an electrical controlconfigured to control a timing of signal pulses.

In some embodiments, the Q-switch further comprises a Pockels cell; anda polarizer in the free-space second portion of the optical signal ringpath on each of two sides of the Pockels cell to linearly polarize thesignal beam on the two sides of the Pockels cell.

In some embodiments, the extraction optics include a polarizing beamsplitter.

Some embodiments further include a first half-wave plate in thefree-space portion of the signal beam path on a first side of thepolarizing beam splitter, and a second half-wave plate in the free-spaceportion of the signal beam path on a first side of the polarizing beamsplitter, wherein the first and second half-wave plates are adjustableto control a proportion of the free-space signal beam that is output,and to align a polarization of a ring-feedback signal beam to that ofthe gain fiber.

In some embodiments, the gain fiber is a polarization-maintaining (PM)gain fiber.

Some embodiments further include a wavelength-converting deviceoptically coupled to receive the intermediate output beam and to converta wavelength of the intermediate output beam to a wavelength differentfrom the wavelength of the intermediate output beam.

In some embodiments, the present invention provides an apparatus thatincludes a fiber gain medium that is configured to form a first portionof an optical signal ring path that extends through the fiber gainmedium such that the optical signal ring path forms a ring laser havinga signal beam; means, as described herein, for optically pumping thefiber gain medium; means, as described herein, for forming a free-spacesignal beam in the optical signal ring path; means, as described herein,for Q-switching the free-space signal beam; and means, as describedherein, for extracting an intermediate output beam from the free-spacesignal beam.

Some embodiments further include means for forcing the signal beam totravel in a first direction around the ring laser.

Some embodiments further include means for filtering the signal beam tolimit a linewidth of the signal beam in the ring laser.

In some embodiments, the means for Q-switching is configured to passlight based on an electrical control signal.

Some embodiments further include a rubidium titanyl phosphate (RTP)Pockels cell; and means for polarizing the signal beam to a linearpolarization on each of two sides of a RTP Pockels cell.

In some embodiments, the means for extracting of the intermediate outputbeam from the free-space beam includes means for polarizing beamsplitter.

Some embodiments further include means for rotating a direction ofpolarization by 90 degrees on both of two sides of the polarizing beamsplitter.

In some embodiments, the means for Q-switching includes means forpassing light based on an electrical control signal, wherein the meansfor extracting of the intermediate output beam from the free-space beamincludes means for polarizing beam splitter, wherein the gain fiber is apolarization-maintaining (PM) gain fiber, and wherein the apparatusfurther includes means for forcing the signal beam to travel in a firstdirection around the ring laser; and means for filtering the signal beamto limit a linewidth of the signal beam in the ring laser.

Some embodiments further include means for wavelength converting theintermediate output beam to a wavelength different from a wavelength ofthe intermediate output beam.

Some embodiments further include means for opto-isolating the signalbeam to travel only in a single direction around the ring; means forfiltering the signal beam to limit a linewidth of the signal beam in thering, wherein the Q-switching includes using a rubidium titanylphosphate (RTP) Pockels cell; means for polarizing the signal beam to alinear polarization on each of two sides of the RTP Pockels cell,wherein the extracting of the intermediate output beam from thefree-space beam includes using a polarizing beam splitter; means forrotating a direction of polarization by 90 degrees on both of two sidesof the polarizing beam splitter, wherein the opto-isolating of thesignal beam is done between the two rotatings of the direction ofpolarization; and means for wavelength converting the intermediateoutput beam to a wavelength different from a wavelength of theintermediate output beam.

Some embodiments of the apparatus described herein further include amaterials-processing unit operably coupled to receive laser outputenergy from one or more of the ring-laser systems and/orwavelength-conversion devices and configured to use the laser outputenergy for materials-processing functions.

In some embodiments, the present invention provides a method thatincludes: providing a fiber gain medium; optically pumping the fibergain medium; outputting a laser signal as an intermediate optical signaloutput beam having a first wavelength from the fiber gain medium;frequency quadrupling the intermediate optical signal output beam toform a frequency-quadrupled optical signal; and transmitting thefrequency-quadrupled optical signal through water, such as seawater.Some embodiments further include encoding the laser signal with data tobe communicated through the water. In some such embodiments, thetransmitting of the signal is between two ships, at least one of whichis a submarine.

Some embodiments further include detecting a light signal caused bylight interaction of the frequency-quadrupled signal with a thermoclinein the water; and processing the detected light signal to derive imageinformation. Some embodiments further include displaying the imageinformation on a monitor.

Some embodiments further include pulsing the laser signal; detecting alight signal from one or more light interactions of the incidentfrequency-quadrupled signal with anomalies in the water (e.g.,scattering, reflections, dispersion and/or the like); and processing thedetected light signal to derive image information. In some suchembodiments, the transmitting of the frequency-quadrupled signal furtherincludes scanning the transmitted frequency-quadrupled signal across arange of angles in order to detect three-dimensional (3D) imageinformation. Some embodiments further include displaying the 3D imageinformation on a 3D monitor. In some such embodiments, the 3D monitorincludes a head-mounted visual-display device for a person, the displayhaving separate displays for each eye of the person. In someembodiments, the 3D monitor includes a large-screen LCD screen thatalternates display frames for the left eye of a viewer with displayframes for the right eye of the viewer, as is well known in the art. Insome embodiments, the 3D monitor includes a large-screen LCD screen thatpresents display frames for the left eye of a viewer with a firstpolarization and simultaneously presents frames for the right eye of theviewer with a different second polarization, such that the 3Dinformation can be viewed by a plurality of persons using polarizedoptics (e.g., polarized glasses having a vertical polarization over theleft eye and horizontal polarization over the right eye (or viceversa)).

In some embodiments, the frequency quadrupling of the intermediateoptical signal output beam further includes: frequency doubling theintermediate optical signal output beam to form a second optical signaloutput beam having a second wavelength that is one-half of the firstwavelength of the intermediate optical signal output beam; and frequencydoubling the second optical signal output beam to form thefrequency-quadrupled optical signal beam having a third wavelength thatis one-half of the second wavelength of the second optical signal outputbeam.

In some embodiments, the fiber gain medium is arranged as a ring laser,and the method further includes forcing the signal beam to travel in afirst direction around the ring laser; forming a free-space signal beamin the ring laser; filtering the signal beam to limit a linewidth of thesignal beam in the ring laser; Q-switching the signal using a rubidiumtitanyl phosphate (RTP) Pockels cell; polarizing the signal beam to alinear polarization on each of two sides of the RTP Pockels cell;extracting the laser signal as an intermediate optical signal outputbeam from the free-space signal beam includes using a polarizing beamsplitter; rotating a direction of polarization by 90 degrees on both oftwo sides of the polarizing beam splitter, wherein the forcing of amajority of the signal beam is done between the two rotatings of thedirection of polarization; and wavelength converting the intermediateoptical signal output beam to a wavelength different from the firstwavelength of the intermediate optical signal output beam.

In some embodiments, the fiber gain medium is arranged as a ring laser,and the method further includes forcing the signal beam to travel in afirst direction around the ring laser; forming a free-space signal beamin the ring laser; filtering the signal beam to limit a linewidth of thesignal beam in the ring laser; amplitude-modulating the signal beam toform pulses; polarizing the signal beam to a linear polarization;extracting the signal beam as an intermediate optical signal output beamfrom the free-space signal beam includes using a polarizing beamsplitter; rotating a direction of polarization on both of two sides ofthe polarizing beam splitter, wherein the forcing of a majority of thesignal beam is done between the two rotatings of the direction ofpolarization; and wavelength converting the intermediate optical signaloutput beam to a wavelength different from the first wavelength of theintermediate optical signal output beam.

Some embodiments further include using the frequency-quadrupled outputbeam for communications through seawater. In some embodiments, thetransmitting of the frequency-quadrupled optical signal is performedfrom a surface vehicle. In some embodiments, the transmitting of thefrequency-quadrupled optical signal is performed from an aircraft. Insome such embodiments the aircraft is an unmanned aerial vehicle (UAV).In some embodiments, the transmitting of the frequency-quadrupledoptical signal is performed from a satellite or other platform locatedat least 100 kilometers from a water surface. In some embodiments, thetransmitting of the frequency-quadrupled optical signal is performedfrom a satellite or other platform located at least 200 kilometers froma water surface. In some embodiments, the transmitting of thefrequency-quadrupled optical signal is performed from a satellite orother platform located at least 300 kilometers from a water surface. Insome embodiments, the transmitting of the frequency-quadrupled opticalsignal is performed from an underwater stationary device tethered to asea bottom. In some embodiments, the transmitting of thefrequency-quadrupled optical signal is performed from a floating buoy.In some embodiments, the transmitting of the frequency-quadrupledoptical signal is performed from a manned underwater vehicle such as asubmarine. In some embodiments, the transmitting of thefrequency-quadrupled optical signal is performed from an unmannedunderwater vehicle (UUV).

Some embodiments further include using the frequency-quadrupled outputbeam for imaging through seawater.

Some embodiments further include using the frequency-quadrupled outputbeam to illuminate underwater features; and detecting and processingreflected light from the frequency-quadrupled output beam to form imagedata. In some embodiments, the method further includes displaying theimage data on a monitor.

Some embodiments further include using the frequency-quadrupled outputbeam for detection and ranging of underwater bodies, includingsubmarines or other underwater vehicles, fish, or marine mammals. Thisis also called light distancing and ranging (LIDAR). In someembodiments, this is done from an aircraft, surface vessel, or submergedvessel.

In some embodiments, the present invention provides an apparatus thatincludes a high-power infrared laser outputting a laser signal having afirst wavelength between 1800 nm and 2000 nm as an intermediate opticalsignal output beam; a data encoder operably coupled to encode data onthe intermediate optical signal output beam; a frequency quadrupleroperably coupled to receive the encoded intermediate optical signaloutput beam and to form a frequency-quadrupled optical signal; and abeam transmitter operably coupled to transmit the frequency-quadrupledoptical signal through water. In other embodiments, the data encoder isinstead configured to encode data on a frequency-doubled beam obtainedby frequency doubling the intermediate optical signal output beam beforethe data is imposed on the beam. In other embodiments, the data encoderis instead configured to encode data on a frequency-quadrupled beamobtained by frequency doubling and frequency doubling again theintermediate optical signal output beam before the data is imposed onthe beam. In some embodiments, the initial (e.g., Tm-doped) fiber laserused to generate the intermediate IR signal uses a master-oscillatorpower-amplifier (MOPA) configuration that uses a seed laser beam from asemiconductor laser that is amplified by one or more Tm-doped fiberamplifiers. In other embodiments, the initial laser (or the seed laserif the initial laser uses a MOPA configuration) is a Q-switched orcavity-dumped ring fiber laser. In yet other embodiments, the seedsource includes a distributed feedback (DFB) laser diode, a distributedBragg reflector (DBR) diode, or a laser diode externally stabilized witha fiber Bragg grating or with a volume Bragg grating.

In some embodiments, the present invention provides an apparatus thatincludes a high-power infrared laser outputting a laser signal having afirst wavelength between 1800 nm and 2000 nm as an intermediate opticalsignal output beam; a pulse generator or a pulse modulator operablycoupled to pulse the intermediate optical signal output beam; afrequency quadrupler operably coupled to receive the pulsed intermediateoptical signal output beam and to form a frequency-quadrupled opticalsignal; a beam transmitter operably coupled to transmit thefrequency-quadrupled optical signal through water; and a light detectorand processor configured to detect and process a sensed light signalfrom light interactions of the incident light signal with anomalies inthe water (e.g., scattering, reflections or the like) from thetransmitted frequency-quadrupled optical signal, to generate 3D imagedata.

In some embodiments, the present invention provides an apparatus thatincludes a fiber gain medium; means for optically pumping the fiber gainmedium; means for outputting a laser signal as an intermediate opticalsignal output beam having a first wavelength from the fiber gain medium;means for frequency quadrupling the intermediate optical signal outputbeam to form a frequency-quadrupled optical signal; and means fortransmitting the frequency-quadrupled optical signal.

Some embodiments further include means for encoding the laser signalwith data to be communicated through the water.

In some embodiments, the means for transmitting of the signal isarranged to communicate data between two ships, at least one of which isa submarine.

Some embodiments further include means for pulsing the laser signal;means for detecting, from the water, a light signal caused by lightinteraction of the frequency-quadrupled signal; and means for processingthe detected light signal to derive image information. In some suchembodiments, the means for transmitting the frequency-quadrupled signalfurther includes means for scanning the transmitted frequency-quadrupledsignal across a range of angles in order to detect three-dimensional(3D) image information.

In some embodiments, the means for frequency quadrupling theintermediate optical signal output beam further includes means forfrequency doubling the intermediate optical signal output beam to form asecond optical signal output beam having a second wavelength that isone-half of the first wavelength of the intermediate optical signaloutput beam; and means for frequency doubling the second optical signaloutput beam to form the frequency-quadrupled optical signal beam havinga third wavelength that is one-half of the second wavelength of thesecond optical signal output beam.

In some embodiments, the fiber gain medium is arranged as a ring laser,the apparatus further comprising means for forcing the signal beam totravel in a first direction around the ring laser; means for forming afree-space signal beam in the ring laser; means for filtering the signalbeam to limit a linewidth of the signal beam in the ring laser; meansfor amplitude-modulating the signal; polarizing the signal beam to alinear polarization; means for extracting the signal beam as anintermediate optical signal output beam from the free-space signal beamincludes using a polarizing beam splitter; means for rotating adirection of polarization on both of two sides of the polarizing beamsplitter, wherein the forcing of a majority of the signal beam is donebetween the two rotatings of the direction of polarization; and meansfor wavelength converting the intermediate optical signal output beam toa wavelength different from the first wavelength of the intermediateoptical signal output beam.

Some embodiments further include means for communicating throughseawater using the frequency-quadrupled output beam. In some suchembodiments, the means for transmitting the frequency-quadrupled opticalsignal operates from a surface vehicle. In some embodiments, the meansfor transmitting the frequency-quadrupled optical signal operates froman aircraft. In some embodiments, the means for transmitting thefrequency-quadrupled optical signal operates from a satellite located atleast 100 kilometers from a water surface. In some embodiments, themeans for transmitting the frequency-quadrupled optical signal operatesfrom an underwater stationary device tethered to a sea bottom. In someembodiments, the means for transmitting the frequency-quadrupled opticalsignal operates from a floating buoy. In some embodiments, the means fortransmitting the frequency-quadrupled optical signal operates from amanned underwater vehicle. In some embodiments, the means fortransmitting the frequency-quadrupled optical signal operates from anunmanned underwater vehicle (UUV).

Some embodiments further include means for imaging through seawaterusing the frequency-quadrupled output beam.

Some embodiments further include means for illuminating underwaterfeatures using the frequency-quadrupled output beam; and means fordetecting and processing reflected light from the frequency-quadrupledoutput beam to form an image.

Some embodiments further include for detection and ranging of underwaterbodies using the frequency-quadrupled output beam.

Some embodiments further include means for imaging disturbances in thethermocline using the frequency-quadrupled output beam.

As used herein, the term “light interaction” includes any change in thedirection, wavelength, phase, spectrum, dispersion, polarization,intensity, and/or other physical property of a propagating lightsignal—it includes both static and dynamic scattering of an incidentlight signal due to any anomaly (including atoms, molecules, changes inindex of refraction (such as might be caused by a thermocline (theregion of relatively abrupt temperature change that resides between theupper mixed layer of water above and the deep ocean water below) orhalocline (abrupt change in salinity)), microscopic life or inanimatedust particles, or by macroscopic bodies such as fish, mammals,arthropods and the like, by underwater objects or underwater vehiclessuch as a submarine, or by the underwater landscape such as sand orunderwater mountains on the sea bottom). Because it includes any changein the direction of a propagating light signal, the term “lightinteraction” as used herein includes what might otherwise be called“reflection.” Because it also includes any change in the wavelength,phase, spectrum or polarization of a propagating light signal, the term“light interaction” as used herein includes what might otherwise becalled “fluorescence,” “changes in the apparent distance ofpropagation,” “absorption” (such as by atomic or molecular species thatselectively absorb more or less of various incident wavelengths),“changes in polarization” or other linear or non-linear effect on thelight signal. These changes arise from (1) the spatial-coherenceproperties of the incident light, (2) the frequency dependence of thepotential (due to dispersion of the medium) and (3) frequency dependenceof the free-space Green function (see Emil Wolf, “Theory of Coherenceand Polarization of light,” Cambridge University Press, New York (2007)pages 111-128, which is incorporated herein by reference). The detectionof a light interaction can be by a light sensor that is located close tothe light source (e.g., for detecting reflections or scattering of thelight signal (or of a wavelength or polarization change to the lightsignal) back toward its source), or by a light sensor that is locatedoff to a side of the direction of propagation of the incident lightsource (for detecting reflections or scattering (or of a wavelength orpolarization change to the light signal) of the light signal in adirection other than back toward its source or toward the same directionas the initial direction of the light signal), or by a light sensorlocated distal and in the same direction as the initial direction of thelight signal (for detecting changes to intensity, wavelength orpolarization due to objects located along a straight line between thesource and the sensor).

As used herein, an “anomaly in the water” means any change in thepresence or relative abundances of atoms or molecules, changes in indexof refraction (such as might be caused by a thermocline or abrupt changein salinity), microscopic life or inanimate dust particles, or bymacroscopic bodies such as fish, mammals, arthropods and the like, byunderwater objects or underwater vehicles such as a submarine, or by theunderwater landscape such as sand or underwater mountains on the seabottom). In some embodiments, the light interaction of the signal beamwith the water surface (a surface signal due, e.g., to reflection orscattering) provides a reference signal (e.g., time-of-flight) fromwhich other interactions with anomalies in the water (submarines,disturbances to the thermocline, halocline, sea bottom, or otheranomalies) are measured with reference to. In some embodiments, thissurface signal is subtracted from other received light-interactionsignal to obtain an improved signal-to-noise ratio of the signal used toderive image data. In some embodiments, the time-of-flight differencesbetween the various intensity (or wavelength, phase, spectrum, orpolarization) features of the sensed signal from the light interactionof the incident light with the various anomalies in the water are usedto generate three-dimensional image data, which can then be eitherautomatically analyzed to detect an object of interest (such as asubmarine), or displayed in various forms (e.g., false color, contourlines, or a rotatable image with simulated reflection or shading to showsurfaces or interfaces, or other convenient form for visualinterpretation by a human user).

It is to be understood that the above description is intended to beillustrative, and not restrictive. Although numerous characteristics andadvantages of various embodiments as described herein have been setforth in the foregoing description, together with details of thestructure and function of various embodiments, many other embodimentsand changes to details will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein,” respectively. Moreover, the terms “first,” “second,” and“third,” etc., are used merely as labels, and are not intended to imposenumerical requirements on their objects.

What is claimed is:
 1. An apparatus comprising: an infrared laseroutputting a laser signal having a first wavelength between 1800 nm and2000 nm as an intermediate optical signal output beam; a frequencyquadrupler operably coupled to receive the intermediate optical signaloutput beam and to output a frequency-quadrupled optical signal; a beampolarizer configured to orient a polarization direction of thefrequency-quadrupled optical signal in a direction that enhancestransmission through an air-water interface; a beam transmitter operablycoupled to transmit the polarized frequency-quadrupled optical signalthrough water; a light detector and processor configured to detect andprocess reflections from the transmitted frequency-quadrupled opticalsignal to generate image data, wherein the light detector includes anarrowband light filter configured to increase a signal-to-noise ratioof the detected reflections.
 2. The apparatus of claim 1, furthercomprising: a data encoder operably coupled to encode data on at leastone of the intermediate optical signal output beam and thefrequency-quadrupled optical signal such that the frequency-quadrupledoptical signal has encoded data.
 3. The apparatus of claim 1, whereinthe infrared laser includes a large-mode-area (LMA) fiber.
 4. Theapparatus of claim 1, wherein the narrowband light filter includes apolarization beamsplitter configured to obtain a plurality of receivedsignals from different polarizations obtained from the beamsplitterincluding a first polarization signal and a second polarization signal,wherein the processor is further configured to process the firstpolarization signal.
 5. The apparatus of claim 4, wherein the processoris further configured to subtract the second polarization signal fromthe first polarization signal to enhance a signal-to-noise ratio of theprocessed received signals.
 6. An apparatus comprising: a fiber gainmedium; means for optically pumping the fiber gain medium; means forgenerating a laser seed signal; means for optically coupling, into thefiber gain medium, the laser seed signal; means for outputting, from thefiber gain medium, an amplified version of the laser seed signal as anintermediate optical signal output beam having a first wavelengthbetween 1800 nm and 2000 nm; means for frequency quadrupling theintermediate optical signal output beam to form a frequency-quadrupledoptical signal; means for orienting a polarization direction of thefrequency-quadrupled optical signal in a direction that enhancestransmission through an air-water interface; means for transmitting thefrequency-quadrupled optical signal through seawater; and means fordetecting, from water, a light signal caused by light interaction of thefrequency-quadrupled signal; and means for processing the detected lightsignal to derive image information.
 7. The apparatus of claim 6, whereinthe means for transmitting the frequency-quadrupled optical signal isarranged to communicate data between two ships, at least one of which isa submarine.
 8. The apparatus of claim 6, wherein the means fortransmitting the frequency-quadrupled signal further includes means forscanning the transmitted frequency-quadrupled signal across a range ofangles in order to detect three-dimensional (3D) image information. 9.The apparatus of claim 6, wherein the means for frequency quadruplingthe intermediate optical signal output beam further includes: means forfrequency doubling the intermediate optical signal output beam to form asecond optical signal output beam having a second wavelength that isone-half of the first wavelength of the intermediate optical signaloutput beam; and means for frequency doubling the second optical signaloutput beam to form the frequency-quadrupled optical signal having athird wavelength that is one-half of the second wavelength of the secondoptical signal output beam.
 10. The apparatus of claim 6, wherein themeans for transmitting the frequency-quadrupled optical signal operatesfrom a surface vehicle.
 11. The apparatus of claim 6, wherein the meansfor transmitting the frequency-quadrupled optical signal operates froman aircraft.
 12. The apparatus of claim 6, further comprising: means forilluminating underwater features using the frequency-quadrupled opticalsignal; and means for detecting and processing reflected light from thefrequency-quadrupled optical signal to form an image.
 13. A methodcomprising: providing a fiber gain medium; optically pumping the fibergain medium; generating a laser seed signal; optically coupling, intothe fiber gain medium, the laser seed signal; outputting, from the fibergain medium, an amplified version of the laser seed signal as anintermediate optical signal output beam having a first wavelengthbetween 1800 nm and 2000 nm; frequency quadrupling the intermediateoptical signal output beam to form a frequency-quadrupled opticalsignal; orienting a polarization direction of the frequency-quadrupledoptical signal in a direction that enhances transmission through anair-water interface; transmitting the frequency-quadrupled opticalsignal through water; and detecting a light signal caused by lightinteraction of the frequency-quadrupled signal with an anomaly in thewater; and processing the detected light signal to derive imageinformation.
 14. The method of claim 13, further comprising: encodingthe laser signal with data to be communicated through the water.
 15. Themethod of claim 14, wherein the transmitting of the signal is betweentwo ships, at least one of which is a submarine.
 16. The method of claim13, further comprising: detecting a light signal caused by lightinteraction of the frequency-quadrupled signal with a thermocline in thewater; and processing the detected light signal to derive imageinformation.
 17. The method of claim 13, wherein the transmitting of thefrequency-quadrupled signal further includes scanning the transmittedfrequency-quadrupled signal across a range of angles in order to detectthree-dimensional (3D) image information.
 18. The method of claim 13,wherein the frequency quadrupling of the intermediate optical signaloutput beam further includes: frequency doubling the intermediateoptical signal output beam to form a second optical signal output beamhaving a second wavelength that is one-half of the first wavelength ofthe intermediate optical signal output beam; and frequency doubling thesecond optical signal output beam to form the frequency-quadrupledoptical signal beam having a third wavelength that is one-half of thesecond wavelength of the second optical signal output beam.
 19. Themethod of claim 13, wherein the transmitting of the frequency-quadrupledoptical signal includes steering a majority of the frequency-quadrupledoptical signal toward a desired target receiver.
 20. The method of claim13, wherein the frequency-quadrupled optical signal has a wavelengththat is set to a wavelength of Fraunhofer feature F.